Client's initial request for a barrel (4)
I believe the answer is:
cask
'a barrel' is the definition.
('cask' can be a synonym of 'barrel')
'client's initial request' is the wordplay.
'initial' suggests taking the first letters.
'request' becomes 'ask' ('ask' can be a synonym of 'request').
The initial letter of 'clients' is 'c'.
'c'+'ask'='CASK'
'for' is the link.
(Other definitions for cask that I've seen before include "Drink here" , "large volume of beer" , "Measure" , "Large barrel" , "Wine vessel; sack (anag.)" .)