He increases the strain (7)
I believe the answer is:
breeder
'he increases' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'strain' is the wordplay.
I cannot quite understand how this works, but
'strain' could be 'breed' (synonyms) and 'breed' 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...
'the' is the link.
Can you help me to learn more?
(Other definitions for breeder that I've seen before include "Siring horse" , "Stud owner" , "Red beer (anag)" , "One producing offspring" , "Type of nuclear reactor - a rabbit is a fast one!" .)