A pain being back in charge, supplying oxygen (7)
I believe the answer is:
aerobic
'a pain being back in charge supplying oxygen' is the definition.
I can't tell whether this definition defines the answer.
'a pain being back in charge' is the wordplay.
'pain' becomes 'bore' (both can mean an annoying person).
'being back' says the letters should be written backwards.
'in charge' becomes 'ic' (abbreviation for 'in charge').
'bore' in reverse letter order is 'erob'.
'a'+'erob'+'ic'='AEROBIC'
Can you help me to learn more?
(Other definitions for aerobic that I've seen before include "Exercises to enhance respiratory efficiency" , "Exercise to improve cardiovascular health" , "Requiring free oxygen to breath" , "(Of exercise) increasing oxygen in blood" , "(Of exercise) improving the use of oxygen" .)