Extremely cold part of journey back I had (5)
I believe the answer is:
gelid
'extremely cold' is the definition.
(I've seen this before)
'part of journey back i had' is the wordplay.
'part of journey' becomes 'leg' ('leg' is part of 'journey').
'back' says the letters should be written in reverse.
'i had' becomes 'id' (I'd).
'leg' back-to-front is 'gel'.
'gel'+'id'='GELID'
(Other definitions for gelid that I've seen before include "Glacial" , "Icy cold - glide (anag)" , "Extremely cold, frozen" .)