Like serous membranes — more than one, by the sound of it (7)
I believe the answer is:
'like serous membranes' is the definition.
(I've seen this before)
'more than one by the sound of it' is the wordplay.
'more than one' becomes 'plural' (I've seen this before).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'plural' is a homophone of 'PLEURAL'.
(Other definitions for pleural that I've seen before include "like serous membranes" , "Like serous membrane" , "Concerning this membrane" , "With respect to an organ" , "Relating to covering of lung" .)