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' indicates one should take the final letters.
'in' means one lot of letters goes inside another.
'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' inserted within '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" .)