Forward March solution
by Kegler

The blank spaces are filled by the names of the four March sisters, as detailed by the extra words’ first letters: THE GIRLS OF “LITTLE WOMEN” BY LOUISA MAY ALCOTT.
In grid order from top to bottom, they are 1: MEG, 2: JO, 3: BETH, and 4: AMY.

Forward March solution grid

Across
1ARTIS + T (SITAR anag) {television}
6AD(A + GI)O {Hessian}
11BEANPOLE (NOBLE APE anag) {enjoys}
12NUTMEG (TUN rev)
13JOB + LESS
15L + ILAC (CALI rev) {governor}
17HA(NSO)MS (SON anag) {intelligence}
18O + N(E)WAY (YAWN rev) {retail}
19C + LEFS (SELF anag) {lyrical}
23SIDE ("SIGHED" hom) {spirit}
24STIR (2 defs) {officer}
26EDEN (NEED anag) {flower}
28MA + CBETH
30DR + ONE {lonely}
31CO(OPUs)P {ink}
33BEA(VE)RS {TVA}
36SU(IT)E {temporarily}
37GIG + GLEe {last}
38GILL (2 defs) {existing}
39SEASHORE (SHOES ARE anag) {wooden}
40AMYLASES ("LAZES" hom)
41MOT + TOS (rev) {observer}
Down
1AB(ALONE)S {massive}
2REJOINED ("RAINED" hom)
3TA(B)LE {excessive}
4IN(L)AWS (SWAIN anag) {nice}
5TOSH (SHOT anag) {being}
6AL'S + ACE {young}
7DE(A)N {leading}
8ANISES (hidden) {oak}
9ITEMS (EMITS anag) {undeniably}
10OMEGAS (even letters)
14L,(OFT)Y {Italian}
16CA(I) + RN {sandstone}
20LOCOS (COOLS anag) {annoying}
21TRAP + EZiES (rev) {mangling}
22FERAL (FLARE anag) {armrests}
25IN UTERO (ROUTINE anag) {yoga}
27NO(V)ELS {adventure}
28MERGES (hidden) {limousine}
29BETHsOUGHT
30DEVIL (rev) {called}
32P(I + L)OT {opponent's}
33BIGAMY (1st letters)
34lEASE {tech}
35S(I)AM {trainee}