Knock over empty potty; headstrong toddler may get it (5,4)
I believe the answer is:
nappy rash
'toddler may get it' is the definition.
'knock over empty potty headstrong' is the wordplay.
'knock' becomes 'pan' (both can mean to criticise**).
'over' says the letters should be written backwards.
'empty' says to hollow out the word (remove centre letters).
'headstrong' becomes 'rash' (I've seen this before).
'potty' with its middle removed is 'py'.
'pan' back-to-front is 'nap'.
'nap'+'py'+'rash'='NAPPY RASH'
(Other definitions for nappy rash that I've seen before include "Baby's discomfort" , "wet spots" .)