Relying on Germany to supply core for reconditioned engine (7)
I believe the answer is:
needing
'relying on' is the definition.
(to rely on something is to need it)
'germany to supply core for reconditioned engine' is the wordplay.
'germany' becomes 'd' (vehicle registration code for Deutschland).
'to supply core for' means one lot of letters goes inside another (some letters go to the core or middle of others).
'reconditioned' is an anagram indicator (the same letters in a new condition).
'engine' with letters rearranged gives 'neeing'.
'd' inserted inside 'neeing' is 'NEEDING'.
(Other definitions for needing that I've seen before include "Wanting" , "Requiring" , "Short of, lacking" , "Eden gin (anag.)" .)