Insect trapper upset married students once (6)
I believe the answer is:
sundew
'insect trapper' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'upset married students once' is the wordplay.
'upset' says the letters should be written in reverse.
'married' becomes 'wed' (wed can mean married).
'students once' becomes 'nus' (I am not sure about this - if you are sure you should believe this answer much more).
'wed'+'nus'='wednus'
'wednus' reversed gives 'SUNDEW'.
Can you help me to learn more?
(Other definitions for sundew that I've seen before include "Insects" , "Insect-eating bog plant" , "Carnivorous plant" , "Insectivorous plant" .)