Vehicle must follow very old terms (5)
I believe the answer is:
vocab
'terms' is the definition.
(short for vocabulary)
'vehicle must follow very old' is the wordplay.
'vehicle' becomes 'cab' (cab is a kind of vehicle).
'must follow' says to put letters next to each other.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'v'+'o'='vo'
'cab' put after 'vo' is 'VOCAB'.
(Other definitions for vocab that I've seen before include "word list" , "List of words" , "Words (informal)" .)