Endeavour to protect old Conservative (4)
I believe the answer is:
tory
'conservative' is the definition.
(informal term for a Conservative Party member)
'endeavour to protect old' is the wordplay.
'endeavour' becomes 'try' (synonyms).
'to protect' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'try' enclosing 'o' is 'TORY'.
(Other definitions for tory that I've seen before include "Traditionalist" , "English Conservative politician or Donegal island" , "Opponent of 8 Down and 14 Across, partner of 14 Down?" , "type on right? Yes" , "blue" .)