One in the capacity of breather for diving apparatus (8)
I believe the answer is:
aqualung
'diving apparatus' is the definition.
(I've seen this before)
'one in the capacity of breather' is the wordplay.
'one' becomes 'a'.
'in the capacity of' becomes 'qua' (I've seen this before).
'breather' becomes 'lung' (I've seen this before).
'a'+'qua'+'lung'='AQUALUNG'
'for' acts as a link.
(Other definitions for aqualung that I've seen before include "Aid for diver" , "Diver's breathing apparatus" , "Diving apparatus" , "Underwater breathing apparatus" , "Form of scuba" .)