Believe me, if you have a hearing impairment . . . (4,2,4)
I believe the answer is:
read my lips
'believe me' is the definition.
(both can be used to say you really mean something)
'you have a hearing impairment' is the wordplay.
I cannot really see how this works, but
'have' could be 'read' (reading is a kind of having) and 'read' is located in the answer.
This explanation may well be incorrect...
'if' acts as a link.
(Other definitions for read my lips that I've seen before include "Listen carefully!" , "Electoral pledge (to deaf audience?)" , "I promise you" .)