In change of gear, metal switched for stone (7)
I believe the answer is:
granite
'stone' is the definition.
(I've seen this before)
'in change of gear metal switched' is the wordplay.
'in' means one lot of letters goes inside another.
'change of' indicates anagramming the letters.
'metal' becomes 'tin' (tin is a kind of metal**).
'switched' is a reversal indicator.
'tin' backwards is 'nit'.
'gear' anagrammed gives 'grae'.
'grae' going around 'nit' is 'GRANITE'.
'for' acts as a link.
(Other definitions for granite that I've seen before include "Hard granular rock" , "Crystalline rock" , "Very hard type of rock" , "Tangier (anag.)" , "Resilience; stone" .)