One finishing school perhaps not a crank (5)
I believe the answer is:
lever
'crank' is the definition.
(thesaurus)
'one finishing school perhaps not a' is the wordplay.
'one finishing school perhaps' becomes 'leaver'.
'not' suggests deleting specific letters.
'leaver' with 'a' taken out is 'LEVER'.
(Other definitions for lever that I've seen before include "Kind of bar for prising or lifting" , "Revel in moving bar" , "Bar for prising something" , "Exert pressure" , "Rigid bar pivoting about a fulcrum" .)