One of five infiltrating network for Arab, say (6)
I believe the answer is:
equine
'for arab say' is the definition.
(arab is a kind of equine)
'one of five infiltrating network' is the wordplay.
'one of five' becomes 'quin' (short for quintuplet, one of five children).
'infiltrating' indicates putting letters inside.
'network' becomes 'EE' (British mobile network EE).
'quin' inserted inside 'ee' is 'EQUINE'.
(Other definitions for equine that I've seen before include "like knight?" , "Horse-like" , "Relating to horses" , "Horsy" , "Like horses" .)