Head of department is member to win over (6)
I believe the answer is:
disarm
'win over' is the definition.
(both can mean to overcome an opponent)
'head of department is member' is the wordplay.
'head of' indicates taking the first letters.
'member' becomes 'arm' (member can mean a limb).
The first letter of 'department' is 'd'.
'd'+'is'+'arm'='DISARM'
'to' is the link.
(Other definitions for disarm that I've seen before include "Render defenceless" , "Remove weapons from" , "Relieve of weapons" , "Mollify or remove weapon from" , "Win the confidence (of)" .)