Go silent, caught by large wildcat in retreat (4,2)
I believe the answer is:
clam up
'go silent' is the definition.
(I've seen this before)
'caught by large wildcat in retreat' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'by' says to put letters next to each other.
'large' becomes 'L' (eg in clothes sizes).
'wildcat' becomes 'puma' (puma is a kind of wildcat).
'in retreat' says the letters should be written backwards.
'puma' backwards is 'amup'.
'c'+'l'+'amup'='CLAM UP'
(Other definitions for clam up that I've seen before include "Refuse to speak" , "Abruptly stop talking (colloq.)" , "Go silent" , "'Fall silent and refuse to talk (4,2)'" , "Refuse to talk like large shellfish?" .)