Unsuitable, popular program likely to entertain Conservative (12)
I believe the answer is:
inapplicable
'unsuitable' is the definition.
(I know that unsuitable can be written as inapplicable)
'popular program likely to entertain conservative' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'program' becomes 'app' (type of computer program).
'likely' becomes 'liable' (similar in meaning).
'to entertain' means one lot of letters goes inside another.
'conservative' becomes 'C' (UK political abbreviation).
'liable' going around 'c' is 'licable'.
'in'+'app'+'licable'='INAPPLICABLE'
(Other definitions for inapplicable that I've seen before include "Not relevant" , "Inappropriate - irrelevant" .)