Lassitude of monkey gulping oxygen (7)
I believe the answer is:
languor
'lassitude' is the definition.
('languor' can be a synonym of 'lassitude')
'monkey gulping oxygen' is the wordplay.
I cannot quite understand how this works, but
'oxygen' could be 'o' (atomic symbol for oxygen) and 'o' is found in the answer.
'monkey' could be 'langur' (langur is a kind of monkey) and 'langur' is present in the leftover letters.
No letters remain.
This may be the basis of clue (or it may be nonsense).
'of' is the link.
Can you help me to learn more?
(Other definitions for languor that I've seen before include "'Lassitude, lethargy (7)'" , "Pleasurable inactivity" , "Pleasant inactivity" , "Dreamy inactivity" , "Lack of energy" .)