Mark caught rodent in school (7)
I believe the answer is:
'mark' is the definition.
(I know that mark can be written as scratch)
'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' (abbreviation).
'crat' going into 'sch' is 'SCRATCH'.
(Other definitions for scratch that I've seen before include "Pull out of match" , "Slight wound - withdraw from contest" , "not much of a handicap" , "score with > no handicap given" , "handle superficial irritation" .)