Leave last bit of coal in small mine (5)
I believe the answer is:
split
'leave' is the definition.
(I've seen this before)
'last bit of coal in small mine' is the wordplay.
'last bit of' says to take the final letters.
'in' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'mine' becomes 'pit' (down the pit).
The last letter of 'coal' is 'l'.
's'+'pit'='spit'
'l' placed inside 'spit' is 'SPLIT'.
(Other definitions for split that I've seen before include "Croatian resort; rend" , "shop" , "Ripped" , "Break open suddenly" , "resort on the Adriatic" .)