Old chaps beginning to see warning signs (5)
I believe the answer is:
omens
'warning signs' is the definition.
(an omen is a bad sign)
'old chaps beginning to see' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'chaps' becomes 'men' (chap means a man).
'beginning to' suggests taking the first letters.
The first letter of 'see' is 's'.
'o'+'men'+'s'='OMENS'
(Other definitions for omens that I've seen before include "Harbingers, portents" , "Prophetic signs of good or evil" , "Prognostications" , "Auguries" , "Signs or warnings for the future" .)