Pressure on gold to turn into chunk (7)
I believe the answer is:
portion
'chunk' is the definition.
(thesaurus)
'pressure on gold to turn into' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'on' indicates putting letters inside (as in clothing \'on\' a person).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'to turn' indicates anagramming the letters.
'into' with letters rearranged gives 'tion'.
'or' inserted into 'tion' is 'ortion'.
'p'+'ortion'='PORTION'
(Other definitions for portion that I've seen before include "allotment" , "Piece of a whole" , "A lot of" , "Helping - fate" , "Serving" .)