One who is barely able to finish the act (8)
I believe the answer is:
stripper
'one who is barely able to finish' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'act' is the wordplay.
I cannot quite understand how this works, but
'act' could be 'strip' (strip is a kind of act) and 'strip' is found in the answer.
The remaining letters 'per' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'the' is the link.
Can you help me to learn more?
(Other definitions for stripper that I've seen before include "Paint remover" , "Exotic dancer" , "Teasing dancer" , "Progressively barer performer" , "Teasing performer" .)