Constant request for pin? (4)
I believe the answer is:
cask
'pin?' is the definition.
(pin can mean a type of cask)
'constant request' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'request' becomes 'ask' (to ask is to request something).
'c'+'ask'='CASK'
'for' acts as a link.
(Other definitions for cask that I've seen before include "Drink here" , "Keg" , "Wine vessel; sack (anag.)" , "Wine barrel" , "Beer container" .)