Spilt beer on the first cap (5)
I believe the answer is:
'cap' is the definition.
(I know that beret is a type of cap)
'spilt beer on the first' is the wordplay.
'spilt' indicates an anagram.
'on' says to put letters next to each other.
'the first' becomes 't' (1st letter of 'the').
'beer' is an anagram of 'bere'.
(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" .)