Old blokes sign here (4)
I believe the answer is:
omen
'sign here' is the definition.
(an omen is a sign)
'old blokes' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'blokes' becomes 'men' (bloke can informally mean a man).
'o'+'men'='OMEN'
(Other definitions for omen that I've seen before include "Sign of things to come" , "Augury, portent" , "premonition" , "Prognostication" , "Foreboding" .)