Compact doesn't seem regular (5)
I believe the answer is:
dense
'compact' is the definition.
(synonyms)
'doesn't seem regular' is the wordplay.
'regular' means one should take alternating letters.
'doesnt'+'seem'='doesntseem'
The alternating letters of 'doesntseem' are 'DENSE'.
(Other definitions for dense that I've seen before include "Thick, impenetrable" , "Needs (anag) -- thick" , "Stupid, crowded together" , "Closely crowded together" , "Close knit" .)