Every now and again recognise ages (4)
I believe the answer is:
eons
'ages' is the definition.
(I know this)
'every now and again recognise' is the wordplay.
'every now and again' means one should take alternating letters.
The alternate letters of 'recognise' are 'EONS'.
(Other definitions for eons that I've seen before include "Divisions of archaeological time" , "Very long periods (US)" , "Divisions of geological time" , "Long periods of time - noes have it" , "Years and years" .)