Soldiers return. Initially not a large wave (6)
I believe the answer is:
signal
'wave' is the definition.
(thesaurus)
'soldiers return initially not a large' is the wordplay.
'soldiers' becomes 'gis' (I've seen this in other clues**).
'return' shows that the letters should be reversed in order.
'initially' says to take the initial letters.
'large' becomes 'l'.
The initial letter of 'not' is 'n'.
'gis' written backwards gives 'sig'.
'sig'+'n'+'a'+'l'='SIGNAL'
(Other definitions for signal that I've seen before include "Remarkable; indication" , "Extraordinary" , "Gesture of communication" , "Nonverbal action or gesture that encodes a message" , "Notable; warning sign" .)