Young lover missing one bird (4)
I believe the answer is:
swan
'bird' is the definition.
(swan is a kind of bird)
'young lover missing one' is the wordplay.
'young lover' becomes 'swain' (I've seen this before).
'missing' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'swain' with 'i' removed is 'SWAN'.
(Other definitions for swan that I've seen before include "Flighty" , "Large, long-necked bird" , "Mute, whooper or Bewick's bird" , "The Ugly Duckling's outcome" , "... Australian river" .)