Text message’s recipient is certainly one taking abnormal interest (6)
I believe the answer is:
usurer
'one taking abnormal interest' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'text message's recipient is certainly' is the wordplay.
I cannot really see how this works, but
'certainly' could be 'sure' (synonyms) and 'sure' is found in the answer.
The remaining letters 'ur' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for usurer that I've seen before include "Unfair moneylender" , "Person who charges excessive interest on loan" , "He expects lots of interest" , "Extortionate moneylender" , "Unscrupulous lender" .)