Fit to bring back Russian space station on time (4)
I believe the answer is:
trim
'fit' is the definition.
(synonyms)
'back russian space station on time' is the wordplay.
'back' says the letters should be written in reverse.
'russian space station' becomes 'mir' (Mir was a Soviet Union space station).
'on' is a charade indicator (letters next to each other).
'time' becomes 't' (abbreviation).
'mir'+'t'='mirt'
'mirt' in reverse letter order is 'TRIM'.
'to bring' acts as a link.
(Other definitions for trim that I've seen before include "(Make) neat and tidy" , "properly organised" , "Dress - prune" , "in good condition" , "Cut neatly; light haircut" .)