He gets terribly involved in the affair (6)
I believe the answer is:
matter
'affair' is the definition.
(I know that affair can be written as matter)
'he gets terribly involved' is the wordplay.
I cannot really see how this works, but
'he' could be 'matt' and 'matt' is found in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'in the' is the link.
Can you help me to learn more?
(Other definitions for matter that I've seen before include "Substance; be important" , "Substance; topic" , "It occupies space, unlike mind or spirit" , "Stuff - mean something" , "Substance - signify" .)