Flap that is holding in large crate (3,6)
I believe the answer is:
tin lizzie
'crate' is the definition.
(both can mean an old car)
'flap that is holding in large' is the wordplay.
'flap' becomes 'tizz' (both can mean a state of nervousness).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'holding' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'tizz'+'ie'='tizzie'
'in'+'l'='inl'
'tizzie' enclosing 'inl' is 'TIN LIZZIE'.
(Other definitions for tin lizzie that I've seen before include "Old car" , "US crate" , "vehicle?" .)