Nick caught rodent in school (7)
I believe the answer is:
scratch
'nick' is the definition.
(I've seen this before)
'caught rodent in school' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'rodent' becomes 'rat' (rat is a kind of rodent).
'in' means one lot of letters goes inside another.
'school' becomes 'sch'.
'c'+'rat'='crat'
'crat' put into 'sch' is 'SCRATCH'.
(Other definitions for scratch that I've seen before include "Lacerate" , "Pull out of match" , "Back out of race" , "Old Nick" , "having superficial injury" .)