Old criminal boarding helicopter is a lummox (10)
I believe the answer is:
clodhopper
'a lummox' is the definition.
(thesaurus)
'old criminal boarding helicopter' is the wordplay.
'criminal' indicates anagramming the letters.
'boarding' means one lot of letters goes inside another.
'helicopter' becomes 'chopper' (synonyms).
'old' with letters rearranged gives 'lod'.
'lod' put into 'chopper' is 'CLODHOPPER'.
'is' is the link.
(Other definitions for clodhopper that I've seen before include "Clumsy person - heavy shoe" , "Clumsy person (wearing heavy shoes?)" , "Lout - heavy boot" , "Hold copper (anag)" , "Clumsy fellow" .)