Jimmy's outside Acton, having dropped off on train (8)
I believe the answer is:
practise
'train' is the definition.
'jimmy's outside acton having dropped off on' is the wordplay.
'jimmy' becomes 'prise' (jimmy can mean to force something open).
'outside' means one lot of letters goes inside another.
'having dropped off' indicates named letters should be taken away.
'acton' with 'on' removed is 'act'.
'prise' going around 'act' is 'PRACTISE'.
(Other definitions for practise that I've seen before include "Drill" , "Observe" , "Custom" , "Contrive by some means" , "Work" .)