What a mousetrap has to be
I believe the answer is:
set
'be' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'what a mousetrap has' is the wordplay.
'has' says the answer is hidden in the clue.
'SET' can be found hidden inside 'what mousetrap'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for set that I've seen before include "group usually found together" , "conventional" , "Major division of tennis match" , "Place - become rigid - adjust - group of tennis games - fixed - clique - fall beneath the horizon" , "Hard; place" .)