Experience (3)
'experience' is the definition.
(synonyms)
This is all the clue.
(Other definitions for undergo that I've seen before include "Endure, experience something" , "put up with" , "experience needed" , "Endure for guerdon" , "Withstand" .)
'experience' is the definition.
(feeling is a kind of experiencing)
This is the entire clue.
(Other definitions for feel that I've seen before include "Have sensation of" , "Experience (an emotion)" , "Perceive, sense" , "16A [JUDGE]" , "Fondle" .)
'experience' is the definition.
(seeing is a kind of experiencing)
This is the entire clue.
(Other definitions for see that I've seen before include "Norwich, for example" , "Watch, equal (a bet)" , "Observe the bishop's domain" , "Appreciate" , "Set eyes on" .)