Drinking last of ale, Hoskins had a thought! (4)
I believe the answer is:
idea
'thought' is the definition.
(idea is a kind of thought)
'drinking last of ale hoskins had a' is the wordplay.
'drinking' means one lot of letters goes inside another.
'last of' indicates one should take the final letters.
'hoskins had' becomes 'I'd' (I had - where the crossword setter is 'Hoskins').
The final letter of 'ale' is 'e'.
'id'+'a'='ida'
'e' inserted into 'ida' is 'IDEA'.
(Other definitions for idea that I've seen before include "Inkling" , "Fancy" , "It comes to mind" , "Something imagined in the mind" , "Sudden inspiration?" .)