Increase the strain (5)
I believe the answer is:
breed
This clue is a double definition.
'increase' is the first definition.
(thesaurus)
'the strain' is the second definition.
('breed' can be a synonym of 'strain')
(Other definitions for breed that I've seen before include "Strain; reproduce" , "Produce offspring, say horses" , "Mate and have offspring" , "Special variety within a species" , "Special variety of domesticated animal" .)