M-master possibly showing this speech defect (7)
I believe the answer is:
stammer
'this speech defect' is the definition.
(I've seen this before)
'm-master possibly' is the wordplay.
'm-master' becomes 'mmaster' ('master' said with a stutter like 'master' was).
'possibly' indicates anagramming the letters.
'mmaster' is an anagram of 'STAMMER'.
'showing' is the link.
(Other definitions for stammer that I've seen before include "Stutter" , "Smart me with defective speech" , "Psychological speech problem" , "Speak with repetition" , "delivery problem" .)