Drop last trace of fat to be highly desirable (4)
I believe the answer is:
plum
'be highly desirable' is the definition.
(plum can mean excellent)
'drop last trace of fat' is the wordplay.
'last' suggests the final letters.
'trace of fat' becomes 'lum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The final letter of 'drop' is 'p'.
'p'+'lum'='PLUM'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for plum that I've seen before include "excellent?" , "murderer" , "Fruit; something choice" , "Oval fruit with single hard stone" , "Shade" .)