Catching duck is not so easy for a squirrel! (7)
I believe the answer is:
hoarder
'squirrel' is the definition.
(squirrels hoard nuts)
'catching duck is not so easy' is the wordplay.
'catching' is an insertion indicator.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'is not so easy' becomes 'harder' ('hard' can be the opposite of 'easy' so perhaps 'harder' is reasonable for 'not so easy').
'o' inserted inside 'harder' is 'HOARDER'.
'for a' acts as a link.
(Other definitions for hoarder that I've seen before include "One gathers" , "Person acting like a squirrel" , "One secretly stores money or valuables" , "Saver" , "Person unlikely to throw things away" .)