Get back group of soldiers with masterstroke (6)
I believe the answer is:
recoup
'get back' is the definition.
(as in recouping funds)
'group of soldiers with masterstroke' is the wordplay.
'group of soldiers' becomes 'RE' (abbreviation for Royal Engineers).
'with' means one lot of letters go next to another.
'masterstroke' becomes 'coup' (I've seen this in another clue).
're'+'coup'='RECOUP'
(Other definitions for recoup that I've seen before include "Get back or recover - say losses" , "Get back (outlay)" , "Make good a loss" , "Recover, losses perhaps" , "Indemnify" .)