Little number donned always? This is the only chance (3,2,5)
I believe the answer is:
now or never
'this is the only chance' is the definition.
('now or never' means something has to be done now)
'little number donned always?' is the wordplay.
'little number' becomes 'no' (abbreviated form of 'number').
'donned' becomes 'worn' (to don an item of clothing is to wear it).
'always?' becomes 'ever' (synonyms).
'no'+'worn'+'ever'='NOW OR NEVER'
(Other definitions for now or never that I've seen before include "very urgent" , "The only chance; It's a Presley song" , "decisive moment?" .)