Lover improved a lair that's too full (9)
I believe the answer is:
overladen
'full' is the definition.
(similar in meaning)
'lover improved a lair that's too' is the wordplay.
'improved' is an anagram indicator.
'lair' becomes 'den' (synonyms).
'that's too' is a charade indicator (letters next to each other).
'lover' with letters rearranged gives 'overl'.
'overl'+'a'+'den'='OVERLADEN'
(Other definitions for overladen that I've seen before include "Carrying excessive amount" , "Bearing excessive load" , "with too much on board" , "too heavy" , "Loaded beyond capacity" .)