Back with vitality in abundance (1,4)
I believe the answer is:
a gogo
'in abundance' is the definition.
(I've seen this before)
'back with vitality' is the wordplay.
'back' becomes 'ago' ('ago' can be a synonym of 'back').
'with' means one lot of letters go next to another.
'vitality' becomes 'go' (I've seen this in other clues).
'ago'+'go'='A GOGO'
(Other definitions for a gogo that I've seen before include "In abundance (informal)" , "in spades" , "In abundance (from Fr.)" , "to one's heart's content" , "Small bell" .)