Growth of Percy's todger (4)
I believe the answer is:
cyst
'growth' is the definition.
(I've seen this before)
'of percy's todger' is the wordplay.
'of' indicates the answer is hidden within the clue.
'CYST' is hidden in the letters of 'percys todger'.
(Other definitions for cyst that I've seen before include "Closed sac in the body, containing fluid" , "pouch" , "Benign growth" , "Abnormal growth" , "Bladder" .)