Don't go to bed yet. Have a small refill (4,2)
I believe the answer is:
stop up
'don't go to bed yet' is the definition.
(to stop up is to stay up late)
'a small refill' is the wordplay.
'a small' becomes 's' (abbreviation - e.g. clothes size).
'refill' becomes 'top up' (I've seen this before).
's'+'topup'='STOP UP'
'have' acts as a link.
(Other definitions for stop up that I've seen before include "Don't go to bed early" , "close" , "Don't retire here" , "Block (eg an earth)" , "Cork" .)