Tramp left in shell (4)
I believe the answer is:
plod
'tramp' is the definition.
(I've seen this before)
'left in shell' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'shell' becomes 'pod' ('pod' can be a synonym of 'shell').
'l' put inside 'pod' is 'PLOD'.
(Other definitions for plod that I've seen before include "Tramp along" , "Copper" , "Trudge wearily" , "Walk like ploughman or policeman?" , "Fuzzy type" .)