Cask is on time to provide a beer (4)
I believe the answer is:
pint
'a beer' is the definition.
'cask is on time' is the wordplay.
'cask' becomes 'pin' (I am not sure about this - if you are sure you should believe this answer much more).
'is on' means one lot of letters go next to another.
'time' becomes 't'.
'pin'+'t'='PINT'
'to provide' is the link.
Can you help me to learn more?
(Other definitions for pint that I've seen before include "Glass of beer (colloq.)" , "Half a quart" , "The working man's beer measure" , "Standard measure of beer or milk" , "Standard measure of Guinness" .)