Cold union may occupy one's lifetime (4)
I believe the answer is:
ague
'cold' is the definition.
(I've seen this before)
'union may occupy one's lifetime' is the wordplay.
I cannot really understand how this works, but
'union' could be 'u' (common abbreviation as in 'Trades Union Congress') and 'u' is found in the answer.
'lifetime' could be 'age' (I've seen this in another clue) and 'age' is found within the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ague that I've seen before include "Malaria, for example" , "Complaint" , "Hot and cold fever" , "Sweating, shivering, fit (once)" , "Fit of the shakes" .)