Correspondent pressure affected plane (3,3)
I believe the answer is:
pen pal
'correspondent' is the definition.
(pen pal is a kind of correspondent)
'pressure affected plane' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'affected' is an anagram indicator.
'plane' is an anagram of 'enpal'.
'p'+'enpal'='PEN-PAL'
(Other definitions for pen pal that I've seen before include "friendly writer" , "A friendly correspondent" , "Friend you know only through correspondence" , "Friend who communicates in letters" , "Mail friend" .)