Sign for old chaps (4)
I believe the answer is:
omen
'sign' is the definition.
(I know that omen is a type of sign)
'old chaps' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'chaps' becomes 'men' (chap can mean a man).
'o'+'men'='OMEN'
'for' acts as a link.
(Other definitions for omen that I've seen before include "Portent, prophetic sign" , "Event regarded as a prophetic sign of good or evil" , "Sign of something about to happen" , "Augur" , "premonition" .)