Spilt beer on the first cap (5)
I believe the answer is:
beret
'cap' is the definition.
(I know that beret is a type of cap)
'spilt beer on the' is the wordplay.
'spilt' indicates an anagram.
'on' says to put letters next to each other.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'beer' anagrammed gives 'bere'.
'bere'+'t'='BERET'
'first' acts as a link.
(Other definitions for beret that I've seen before include "cover for commandos" , "Frenchman's flat hat" , "Cap with no brim" , "Soft round cap without a peak" , "Round flattish cap" .)