Be mournful when member is in middle of speech (7)
I believe the answer is:
elegise
'be mournful' is the definition.
'member is in middle of speech' is the wordplay.
'member' becomes 'leg' (I've seen this before).
'in' is an insertion indicator.
'middle of' says to take the centre.
The centre of 'speech' is 'ee'.
'leg'+'is'='legis'
'legis' inserted within 'ee' is 'ELEGISE'.
'when' is the link.
(Other definitions for elegise that I've seen before include "To write a funeral song" , "Write mournfully" .)