Spike for working below mine (5)
I believe the answer is:
piton
'spike' is the definition.
(piton is a kind of spike)
'working below mine' is the wordplay.
'working' becomes 'on'.
'below' means one lot of letters go next to another.
'mine' becomes 'pit' (down the pit).
'on' after 'pit' is 'PITON'.
'for' acts as a link.
(Other definitions for piton that I've seen before include "something upwardly mobile type hammers home" , "In top (anag.)" , "Kind of rope-supporting peg used in rock-climbing" , "Peg used in mountaineering" , "Climber's tool" .)