Old man in French capital needs us back together (4,5)
I believe the answer is:
pari passu
'together' is the definition.
(I've seen this before)
'old man in french capital needs us back' is the wordplay.
'old man' becomes 'pa' (both can mean father).
'in' indicates putting letters inside.
'french capital' becomes 'paris' (synonyms).
'needs' is a charade indicator (letters next to each other).
'back' says the letters should be written in reverse.
'us' backwards is 'su'.
'pa' going into 'paris' is 'paripas'.
'paripas'+'su'='PARI PASSU'
(Other definitions for pari passu that I've seen before include "Simultaneously" , "Together" .)