Growth of gang limited in battle (9)
I believe the answer is:
accretion
'growth' is the definition.
(thesaurus)
'gang limited in battle' is the wordplay.
'gang' becomes 'crew' ('crew' can be a synonym of 'gang').
'limited' means to remove the last letter.
'in' means one lot of letters goes inside another.
'battle' becomes 'action' (military action).
'crew' with its last letter removed is 'cre'.
'cre' placed into 'action' is 'ACCRETION'.
'of' acts as a link.
(Other definitions for accretion that I've seen before include "Extra amount" , "Gradual growth, increase" , "Something added" , "extra part" .)