Fish snare upset children (5)
I believe the answer is:
tench
'fish' is the definition.
(tench is a kind of fish)
'snare upset children' is the wordplay.
'snare' becomes 'net' (I've seen this in another clue).
'upset' is a reversal indicator.
'children' becomes 'ch' (genealogical abbreviation).
'net' backwards is 'ten'.
'ten'+'ch'='TENCH'
(Other definitions for tench that I've seen before include "something fishy" , "One in school" , "catch" , "Coarse fish" , "Fish of the carp family" .)