Final act of pallid lad in extremes of suffering (4,4)
I believe the answer is:
swan song
'final act' is the definition.
(swan song can mean a final performance or act)
'pallid lad in extremes of suffering' is the wordplay.
'pallid' becomes 'wan' (synonyms).
'lad' becomes 'son' (lad can informally mean a young boy).
'in' indicates putting letters inside.
'extremes of' suggests removing the centre (only the letters at the extremes of the word).
'suffering' with its middle removed is 'sg'.
'wan'+'son'='wanson'
'wanson' placed inside 'sg' is 'SWAN-SONG'.
'of' acts as a link.
(Other definitions for swan song that I've seen before include "Musician's final appearance" , "Musician's last work" , "A last performance, as of noble bird" , "Last appearance" , "A dying performance" .)