Your move at first to join our side? You’ll get it in the neck! (6)
I believe the answer is:
thymus
'you'll get it in the neck' is the definition.
(organ in the neck)
'your move at first to join our side?' is the wordplay.
'your' becomes 'thy' ('thy' historically meant 'your').
'at first' says to take the initial letters.
'to join' is a charade indicator (letters next to each other).
'our side?' becomes 'us' (our people or our side are us).
The first letter of 'move' is 'm'.
'thy'+'m'+'us'='THYMUS'
(Other definitions for thymus that I've seen before include "A gland in neck" , "Organ" .)