Atomic group's spy meets university lecturer in vehicle (9)
I believe the answer is:
molecular
'atomic' is the definition.
(thesaurus)
'spy meets university lecturer in vehicle' is the wordplay.
'spy' becomes 'mole' (I've seen this before).
'meets' says to put letters next to each other.
'university lecturer' becomes 'ul' (this might be a standard abbreviation which I don't know about).
'in' means one lot of letters goes inside another.
'vehicle' becomes 'car' (car is a kind of vehicle).
'ul' going within 'car' is 'cular'.
'mole'+'cular'='MOLECULAR'
'group's' acts as a link.
This may not be correct. It may be part of another bit of the clue.
(Other definitions for molecular that I've seen before include "Dealing with small particles" , "Relating to the smallest particles of a substance" , "Relating to groups of atoms bonded together" .)