Biscuity food from first of restaurants overlooking Welsh river (4)
I believe the answer is:
rusk
'biscuity food' is the definition.
(rusk is a type of biscuit)
'first of restaurants overlooking welsh river' is the wordplay.
'first of restaurants' becomes 'r' (1st letter of 'restaurants').
'overlooking' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'welsh river' becomes 'usk' (River Usk in Wales).
'r'+'usk'='RUSK'
'from' acts as a link.
(Other definitions for rusk that I've seen before include "Hard crisp baby food" , "cake" , "Small hard biscuit (for babies)" , "Bread baked for babies" , "in Bristol Channel" .)