The only lightweight hat left? This is too much! (4,5)
I believe the answer is:
last straw
'left? this is too much' is the definition.
The definition suggests a singular noun which matches the answer.
'the only lightweight hat' is the wordplay.
'the only lightweight' becomes 'last' (I can't explain this - if you can you should believe this answer much more).
'hat' becomes 'straw' (I've seen this in another clue).
'last'+'straw'='LAST STRAW'
Can you help me to learn more?
(Other definitions for last straw that I've seen before include "The final irritation that stretches your patience beyond the limit" , "camel's killer" , "Downfall of the laden camel" , "The end of the harvest" , "The one that breaks the camel's back" .)