Make a hole in second rock that may hold gold (4)
I believe the answer is:
bore
'make a hole' is the definition.
(I've seen this in another clue)
'second rock that may hold gold' is the wordplay.
'second' becomes 'b' (B can mean second eg B-side).
'rock that may hold gold' becomes 'ore' (metal might be extracted from an ore).
'b'+'ore'='BORE'
'in' acts as a link.
(Other definitions for bore that I've seen before include "I go on" , "Irritate" , "Make a hole with a pointed tool" , "Dull person; use a drill" , "Pierce - weary - tidal flood" .)