Old politician's hot in synthetic headgear (4)
I believe the answer is:
whig
'old politician's' is the definition.
(I've seen this before)
'hot in synthetic headgear' is the wordplay.
'hot' becomes 'h'.
'in' means one lot of letters goes inside another.
'synthetic headgear' becomes 'wig' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'h' going inside 'wig' is 'WHIG'.
Can you help me to learn more?
(Other definitions for whig that I've seen before include "Supporter of the British political party that preceded the Liberals" , "Supporter of an old English political party" , "Member of a former reforming party" , "Former 14 Down" , "Old politician's" .)