Inactivity of monkey without oxygen (7)
I believe the answer is:
languor
'inactivity' is the definition.
(languor is a kind of inactivity)
'monkey without oxygen' is the wordplay.
'monkey' becomes 'langur' (langur is a kind of monkey).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'oxygen' becomes 'o' (atomic symbol for oxygen).
'langur' going around 'o' is 'LANGUOR'.
'of' acts as a link.
(Other definitions for languor that I've seen before include "Pleasurable inactivity" , "'Lassitude, lethargy (7)'" , "Lack of vitality, listlessness" , "No enthusiasm" , "Feeling of boredom or feebleness" .)