One arriving to secure position as server, say (8)
I believe the answer is:
computer
'server say' is the definition.
(server is a type of computer)
'one arriving to secure position' is the wordplay.
'one arriving' becomes 'comer' (someone coming).
'to secure' indicates putting letters inside (inserted letters are secured inside).
'position' becomes 'put' (synonyms).
'comer' placed around 'put' is 'COMPUTER'.
'as' acts as a link.
(Other definitions for computer that I've seen before include "PC or Mac?" , "Eg, PC" , "Laptop, perhaps" , "Eg, laptop" , "Machine for calculating and storing data" .)