Feeling it's a key to the proposal (7)
I believe the answer is:
emotion
'feeling' is the definition.
(emotion is a kind of feeling)
'a key to the proposal' is the wordplay.
'a key' becomes 'e' (I've seen this in other clues).
'to the proposal' becomes 'motion' (motion is a kind of proposal).
'e'+'motion'='EMOTION'
'it's' is the link.
(Other definitions for emotion that I've seen before include "Intuitive feeling" , "A strong instinctive feeling" , "Agitation of mind" , "Feeling like love or fear" , "E.g. fear, hate etc." .)