He toiled to reset instrument (10)
I believe the answer is:
theodolite
'instrument' is the definition.
(I know that theodolite is a type of instrument)
'he toiled to reset' is the wordplay.
'he' becomes 'theo' ().
'to reset' indicates anagramming the letters.
'toiled' with letters rearranged gives 'dolite'.
'theo'+'dolite'='THEODOLITE'
(Other definitions for theodolite that I've seen before include "Surveyor's instrument" , "transit, in America" , "Surveying instrument" , "What a surveyor might use" .)