Weakens, yes, but it's backed by reserves (5)
I believe the answer is:
tires
'weakens yes' is the definition.
'tires' can be an answer for 'weakens' (I've seen this before). I'm not sure about the 'yes' bit.
'it's backed by reserves' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'backed by reserves' becomes 'ires' (I can't justify this - if you can you should give a lot more credence to this answer).
't'+'ires'='TIRES'
'but' is the link.
Can you help me to learn more?
(Other definitions for tires that I've seen before include "Wearies" , "Weakens" , "Becomes weary and in need of rest" , "Exhausts" , "Grows weary of rites" .)