Formal warning for a minor offence (7)
I believe the answer is:
caution
'formal warning for a minor offence' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for caution that I've seen before include "Warn strongly" , "Attention to possible danger" , "Admonish - warning" , "Formal warning for a minor offence" , "A surprising person" .)