Supposed reason purser occasionally given message (7)
I believe the answer is:
pretext
'supposed reason' is the definition.
(I've seen this in another clue)
'purser occasionally given message' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'given' means one lot of letters go next to another.
'message' becomes 'text' (text message).
The alternating letters of 'purser' are 'pre'.
'pre'+'text'='PRETEXT'
(Other definitions for pretext that I've seen before include "Ostensible reason" , "Specious excuse" , "Fictitious excuse" , "Alleged reason" , "Excuse for an action" .)