Dad's keeping half of ugly growths (6)
I believe the answer is:
polyps
'growths' is the definition.
(polyp is a kind of growth)
'dad's keeping half of ugly' is the wordplay.
'dad's' becomes 'pop's' ('pop' can be a synonym of 'dad').
'keeping' indicates putting letters inside (some letters kept inside).
'half of' suggests halving the letters.
'ugly' cut in half is 'ly'.
'pops' placed around 'ly' is 'POLYPS'.
(Other definitions for polyps that I've seen before include "Such as sea anemones" , "sea anemone forms" , "Organisms of which coral is composed" , "Small growths on mucous membrane as in nose" .)