One isn't put out by it but it's a strain (7)
I believe the answer is:
tension
'it's a strain' is the definition.
('tension' can be a synonym of 'strain')
'one isn't put out' is the wordplay.
'put out' is an anagram indicator.
'one'+'isnt'='oneisnt'
'oneisnt' with letters rearranged gives 'TENSION'.
'by it but' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tension that I've seen before include "Mental or emotional strain" , "Strained state" , "nerves" , "Uneasiness" , "Stress, tautness" .)