Rookie fawn's trapped in river next to city centre (7)
I believe the answer is:
recruit
'rookie' is the definition.
(thesaurus)
'fawn's trapped in river next to city centre' is the wordplay.
'fawn' becomes 'ecru' (I've seen this before).
'trapped in' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'next to' says to put letters next to each other.
'city centre' becomes 'it' (centre of the word 'city').
'r'+'it'='rit'
'ecru' placed within 'rit' is 'RECRUIT'.
(Other definitions for recruit that I've seen before include "Newly enlisted person" , "Recently enlisted person" , "Sign up" , "New entrant as to the army or police" , "He's just joined up, often the armed forces" .)