Ungentlemanly spilling of a pint (5)
I believe the answer is:
inapt
'ungentlemanly' is the definition.
(both can mean inappropriate or unseemly)
'spilling of a pint' is the wordplay.
'spilling of' indicates an anagram (I've seen 'spills' mean this).
'a'+'pint'='apint'
'apint' is an anagram of 'INAPT'.
(Other definitions for inapt that I've seen before include "Unsuited" , "unqualified" , "Not fit, unsuitable" , "Out of place" , "Unsuitable - paint (anag)" .)