Intellect welcoming valentine's lips, just romance (4,6)
I believe the answer is:
love affair
'romance' is the definition.
(synonyms)
'intellect welcoming valentine's lips just' is the wordplay.
'intellect' becomes 'loaf' (both can mean one's mind or brain).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'lips' suggests removing the centre (only the edges or lips of the word).
'just' becomes 'fair' (just can mean fair in the sense of showing justice).
'valentine' with its middle removed is 've'.
'loaf' placed around 've' is 'loveaf'.
'loveaf'+'fair'='LOVE-AFFAIR'
(Other definitions for love affair that I've seen before include "Romance" , "A (short?) romantic relationship" .)