Star necking bit of temazepam gets surprise (4)
I believe the answer is:
stun
'surprise' is the definition.
(stunning is a kind of surprising)
'star necking bit of temazepam' is the wordplay.
'star' becomes 'sun' (the Sun is a star).
'necking' indicates putting letters inside (neck can mean to swallow).
'bit of' suggests taking the first letters.
The initial letter of 'temazepam' is 't'.
'sun' enclosing 't' is 'STUN'.
'gets' acts as a link.
(Other definitions for stun that I've seen before include "Stagger, amaze" , "'Shock, daze (4)'" , "Numb with nuts" , "Confound" , "Concuss; amaze" .)