Bulge throbbing softly (4)
I believe the answer is:
hump
'bulge' is the definition.
(I know that bulge can be written as hump)
'throbbing softly' is the wordplay.
'throbbing' becomes 'hum' (I can't explain this - if you can you should give a lot more credence to this answer).
'softly' becomes 'p' (short for piano, musical term for softly).
'hum'+'p'='HUMP'
Can you help me to learn more?
(Other definitions for hump that I've seen before include "Carry (heavy loads) (informal)" , "What you get" , "Rounded protuberance" , "Decorative edging" , "Part of a camel" .)