Hide constant desire (5)
I believe the answer is:
cache
'hide' is the definition.
(I've seen this in another clue)
'constant desire' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'desire' becomes 'ache' (ache can mean a passionate desire for something).
'c'+'ache'='CACHE'
(Other definitions for cache that I've seen before include "Hidden storage place" , "Hidden storage space" , "Hiding place for treasure or store" , "Hiding-place for treasure etc" , "Secret store of valuables" .)