Boxers, for example, perform with good seconds (4)
I believe the answer is:
dogs
'boxers for example' is the definition.
(boxer is a kind of dog)
'perform with good seconds' is the wordplay.
'perform' becomes 'do' (both can mean complete an action).
'with' means one lot of letters go next to another.
'good' becomes 'g' (abbreviation).
'seconds' becomes 's'.
'do'+'g'+'s'='DOGS'
(Other definitions for dogs that I've seen before include "Pursues" , "eg 27 terriers" , "Setters, for example" , "Persistently follows" , "Shadows" .)