Dull dog writhing in pen (6)
I believe the answer is:
stodgy
'dull' is the definition.
('stodgy' can be a synonym of 'dull')
'dog writhing in pen' is the wordplay.
'writhing' is an anagram indicator.
'in' indicates putting letters inside.
'pen' becomes 'sty' (sty is a kind of pen).
'dog' with letters rearranged gives 'odg'.
'odg' put within 'sty' is 'STODGY'.
(Other definitions for stodgy that I've seen before include "Thick and boring (of food)" , "Dull and heavy" , "Heavy and sticky" , "Describes excessively starchy food" , "(Of food) solid and heavy" .)