Volunteers in silent change (6)
I believe the answer is:
mutate
'change' is the definition.
(mutating is a kind of changing)
'volunteers in silent' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'in' indicates putting letters inside.
'silent' becomes 'mute' (synonyms).
'ta' going into 'mute' is 'MUTATE'.
(Other definitions for mutate that I've seen before include "Change in basic nature" , "Convert" , "(In genetics) change" , "Undergo alteration in form" , "Change genetically" .)