Yawning conservative to declare common sense (9)
I believe the answer is:
cavernous
'yawning' is the definition.
(I've seen this in another clue)
'conservative to declare common sense' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'to declare' becomes 'aver' (averring is a kind of declaring).
'common sense' becomes 'nous' (nous is a kind of common sense).
'c'+'aver'+'nous'='CAVERNOUS'
(Other definitions for cavernous that I've seen before include "of large underground chambers" , "Roomy" , "Gaping" , "like a big pit" .)