An age that's critical during dry season abroad (7,5)
I believe the answer is:
donkeys years
'an age' is the definition.
(I have seen 'An age' mean 'donkeys years' so perhaps 'age' could also mean 'donkeys years')
'critical during dry season abroad' is the wordplay.
'critical' becomes 'key' (I've seen this in other clues).
'during' means one lot of letters goes inside another.
'abroad' indicates anagramming the letters.
'dry'+'season'='dryseason'
'dryseason' is an anagram of 'donsyears'.
'key' put inside 'donsyears' is 'DONKEYS YEARS'.
'that's' acts as a link.
(Other definitions for donkeys years that I've seen before include "forever" , "What's evident in old ass" , "A beastly long time" , "Ages" , "Yonks" .)