Spilt beer on top of the cap (5)
I believe the answer is:
beret
'cap' is the definition.
(I know that beret is a type of cap)
'spilt beer on top of the' is the wordplay.
'spilt' is an anagram indicator.
'on top of' indicates taking the first letters.
The initial letter of 'the' is 't'.
'beer' anagrammed gives 'bere'.
'bere'+'t'='BERET'
(Other definitions for beret that I've seen before include "Flattish round hat" , "item of headwear" , "Cap with no brim" , "Flat French cap" , "Kind of floppy cap with no peak" .)