Pressure on parent with son's disease (5)
I believe the answer is:
mumps
'disease' is the definition.
(mumps is a kind of disease)
'pressure on parent with son's' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'on' says to put letters next to each other.
'parent' becomes 'mum' (mum means a mother).
'with' is a charade indicator (letters next to each other).
'p' put after 'mum' is 'mump'.
'mump'+'s'='MUMPS'
(Other definitions for mumps that I've seen before include "harmful disease" , "Infectious glandular disease" , "Epidemic parotitis" , "Virus disease causing painful swellings in neck" , "Swollen-face disease" .)