Stunning boyfriend by returning fit, with bulk stripped down (9)
I believe the answer is:
beautiful
'stunning' is the definition.
(I've seen this in another clue)
'boyfriend by returning fit with bulk stripped down' is the wordplay.
'boyfriend' becomes 'beau' (synonyms).
'by returning fit' becomes 'tif' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' is a charade indicator (letters next to each other).
'stripped down' says to take the centre.
The middle of 'bulk' is 'ul'.
'beau'+'tif'+'ul'='BEAUTIFUL'
Can you help me to learn more?
(Other definitions for beautiful that I've seen before include "Aesthetically very pleasing" , "Very pleasant to behold" , "Pleasing to the eye" , "Very pleasing to the senses" , "Fine" .)