Unwell, and so weak in the end (4)
I believe the answer is:
sick
'unwell' is the definition.
('sick' can be similar in meaning to 'unwell')
'so weak in the end' is the wordplay.
'so' becomes 'sic' ('sic' can be a synonym of 'so'**).
'in the end' says to take the final letters.
The final letter of 'weak' is 'k'.
'sic'+'k'='SICK'
'and' acts as a link.
(Other definitions for sick that I've seen before include "Ill; perverted" , "vomit" , "well? Hardly" , "Infirm" , "Poorly" .)