Handsome boyfriend gets flu - it spread (9)
I believe the answer is:
beautiful
'handsome' is the definition.
(similar in meaning)
'boyfriend gets flu it spread' is the wordplay.
'boyfriend' becomes 'beau' (synonyms).
'gets' means one lot of letters goes inside another.
'spread' indicates an anagram.
'flu'+'it'='fluit'
'fluit' anagrammed gives 'tiful'.
'beau' put into 'tiful' is 'BEAUTIFUL'.
(Other definitions for beautiful that I've seen before include "Aesthetically very pleasing" , "Very pleasant to behold" , "Visually pleasing" , "Nice to see" , "Lovely" .)