Fellow politician backed Number Ten, exercising constituent (9)
I believe the answer is:
component
'constituent' is the definition.
(synonyms)
'fellow politician backed number ten exercising' is the wordplay.
'fellow' becomes 'co' (eg a co-worker is a fellow worker).
'politician' becomes 'MP' (Member of Parliament).
'backed' says the letters should be written backwards.
'number' becomes 'no' (abbreviation).
'exercising' is an anagram indicator (letters exercise or move around).
'no' backwards is 'on'.
'ten' with letters rearranged gives 'ent'.
'co'+'mp'+'on'+'ent'='COMPONENT'
(Other definitions for component that I've seen before include "Part of a whole" , "Element of assembly, elec. circuit" , "Element of a larger whole" , "Constituent part" , "core part" .)