Quietly must leave golf club, say (5)
I believe the answer is:
utter
'say' is the definition.
(I've seen this before)
'quietly must leave golf club' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'must leave' is a deletion indicator.
'golf club' becomes 'putter' (I've seen this before).
'putter' with 'p' removed is 'UTTER'.
(Other definitions for utter that I've seen before include "Total < state" , "Make sounds (but not necessarily words)" , "Thoroughgoing" , "outright" , "Express audibly" .)