Vigorous soldiers back exercising (7)
I believe the answer is:
rousing
'vigorous' is the definition.
(I've seen this in another clue)
'soldiers back exercising' is the wordplay.
'soldiers' becomes 'OR' (Other Ranks).
'back' is a reversal indicator.
'exercising' becomes 'using' (I've seen this before).
'or' reversed gives 'ro'.
'ro'+'using'='ROUSING'
(Other definitions for rousing that I've seen before include "becomes animated" , "Stirring" , "Sour gin (anag.)" .)