Quizzes police sergeant about Unionist politician (5)
I believe the answer is:
pumps
'quizzes' is the definition.
'police sergeant about unionist politician' is the wordplay.
'police sergeant' becomes 'ps'.
'about' means one lot of letters goes inside another.
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'politician' becomes 'mp'.
'u'+'mp'='ump'
'ps' enclosing 'ump' is 'PUMPS'.
(Other definitions for pumps that I've seen before include "needed by cyclists" , "Light dancing shoes - force air into" , "Shoes; inflaters" , "Dancing shoes - may be manned!" , "footwear" .)