Most enthusiastic expression of surprise over cosy retreat (7)
I believe the answer is:
keenest
'most enthusiastic' is the definition.
(I've seen this before)
'expression of surprise over cosy retreat' is the wordplay.
'expression of surprise' becomes 'eek' (I've seen this before).
'over' is a reversal indicator.
'cosy retreat' becomes 'nest' (I've seen this before).
'eek' written backwards gives 'kee'.
'kee'+'nest'='KEENEST'
(Other definitions for keenest that I've seen before include "Most eager and sharp" , "Most interested or sharp" , "most sensitive" , "Most anxious" , "Sharpest" .)