Rear enthusiast to cross river followed by another (7)
I believe the answer is:
nurture
'rear' is the definition.
(synonyms)
'enthusiast to cross river followed by another' is the wordplay.
'enthusiast' becomes 'nut' (I've seen this before).
'to cross' means one lot of letters goes inside another.
'river' becomes 'r'.
'followed' says to put letters next to each other.
'by another' becomes 'ure' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'nut' placed around 'r' is 'nurt'.
'nurt'+'ure'='NURTURE'
(Other definitions for nurture that I've seen before include "Care for during upbringing" , "Training" , "Run true (anag)" , "Parenting" , "Feed - support" .)