Firm on leaving later today (5)
I believe the answer is:
tight
'firm' is the definition.
(synonyms)
'on leaving later today' is the wordplay.
'leaving' suggests deleting specific letters.
'later today' becomes 'tonight' (I've seen this before).
'tonight' with 'on' removed is 'TIGHT'.
(Other definitions for tight that I've seen before include "Close-fitting; tense" , "lack of money" , "Fast - affected by drink" , "Strictly imposed; drunk" , "Bottled ," .)