Terrible ladies' man, sort to show ineptitude (13)
I believe the answer is:
maladroitness
'ineptitude' is the definition.
('maladroitness' can be a synonym of 'ineptitude')
'terrible ladies' man sort' is the wordplay.
'terrible' is an anagram indicator.
'ladies'+'man'+'sort'='ladiesmansort'
'ladiesmansort' anagrammed gives 'MALADROITNESS'.
'to show' is the link.
(Other definitions for maladroitness that I've seen before include "Clumsiness" , "insensitivity" , "Clumsy behaviour" .)