Feeling of love appearing in no time, unexpectedly (7)
I believe the answer is:
emotion
'feeling' is the definition.
(emotion is a kind of feeling)
'love appearing in no time unexpectedly' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'appearing in' is an insertion indicator.
'unexpectedly' is an anagram indicator.
'no'+'time'='notime'
'notime' with letters rearranged gives 'emotin'.
'o' put within 'emotin' is 'EMOTION'.
'of' is the link.
(Other definitions for emotion that I've seen before include "Mental agitation" , "Mental feeling" , "Strong desire" , "Intuitive feeling" , "Hatred or love, say" .)