Declare golf club lacks power (5)
I believe the answer is:
utter
'declare' is the definition.
(I know that declare is a more specific form of say)
'golf club lacks power' is the wordplay.
'golf club' becomes 'putter' (type of golf club).
'lacks' suggests deleting specific letters.
'power' becomes 'p' (symbol in physics).
'putter' with 'p' taken away is 'UTTER'.
(Other definitions for utter that I've seen before include "Prize" , "Say - complete" , "Complete madman" , "Speak, express" , "Express in speech" .)