Treacherous type regularly wanting in on correspondence (5)
I believe the answer is:
ratio
'correspondence' is the definition.
(thesaurus)
'treacherous type regularly wanting in on' is the wordplay.
'treacherous type' becomes 'rat' (rat can mean a treacherous person).
'regularly wanting' indicates alternate letters (letters removed at regular intervals).
'in'+'on'='inon'
The alternate letters of 'inon' are 'io'.
'rat'+'io'='RATIO'
(Other definitions for ratio that I've seen before include "Quotient, proportion" , "E.g. two to one" , "Proportion, relationship" , "eg 1:3" , "Comparative measure" .)