Clumsy person, terribly old, boarding helicopter (10)
I believe the answer is:
clodhopper
'clumsy person' is the definition.
(I've seen this before)
'terribly old boarding helicopter' is the wordplay.
'terribly' is an anagram indicator.
'boarding' is an insertion indicator.
'helicopter' becomes 'chopper' (synonyms).
'old' anagrammed gives 'lod'.
'lod' going within 'chopper' is 'CLODHOPPER'.
(Other definitions for clodhopper that I've seen before include "Foolish person; large heavy shoe" , "bumpkin" , "yokel" , "Hold copper (anag)" , "Lout - heavy boot" .)