Shellfish with feathers bringing in start of pesticide restriction (9)
I believe the answer is:
clampdown
'restriction' is the definition.
(clampdown is a kind of restriction)
'shellfish with feathers bringing in start of pesticide' is the wordplay.
'shellfish with' becomes 'clam' (I can't justify this - if you can you should give a lot more credence to this answer).
'feathers' becomes 'down' (down is a kind of feathers).
'bringing in' is an insertion indicator.
'start of' says to take the initial letters.
The initial letter of 'pesticide' is 'p'.
'clam'+'down'='clamdown'
'clamdown' going around 'p' is 'CLAMPDOWN'.
Can you help me to learn more?
(Other definitions for clampdown that I've seen before include "Sudden restriction on an activity" , "restrictive measure" , "Restrictive practice" , "suppression" , "stopper?" .)