Unmanageable soldiers, spanner finally thrown in works (10)
I believe the answer is:
refractory
'unmanageable' is the definition.
(thesaurus)
'soldiers spanner finally thrown in works' is the wordplay.
'soldiers' becomes 'RE' (Royal Engineers).
'finally' says to take the final letters.
'thrown in' means one lot of letters goes inside another.
'works' becomes 'factory' (factory is a kind of works).
The final letter of 'spanner' is 'r'.
'r' placed inside 'factory' is 'fractory'.
're'+'fractory'='REFRACTORY'
(Other definitions for refractory that I've seen before include "recalcitrant" , "Difficult to control" , "Disobedient - uncontrollable" , "Racer, forty (anag.)" , "Obstinate, wilful" .)