Volunteers fast for gift (6)
I believe the answer is:
talent
'for gift' is the definition.
(synonyms)
'volunteers fast' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'ta'+'lent'='TALENT'
(Other definitions for talent that I've seen before include "Natural ability or aptitude" , "Old coin for natural ability" , "Special or very great ability" , "Natural aptitude of an old coin" , "Natural endowment" .)