Spies capturing tunnel all returned needing oxygen (7)
I believe the answer is:
aerobic
'needing oxygen' is the definition.
(I've seen this in another clue)
'spies capturing tunnel all returned' is the wordplay.
'spies' becomes 'CIA' (Central Intelligence Agency).
'capturing' means one lot of letters goes inside another.
'tunnel all' becomes 'bore' (I've seen this in another clue. I am not sure about the 'all' bit.).
'returned' says the letters should be written in reverse.
'cia' going around 'bore' is 'ciborea'.
'ciborea' backwards is 'AEROBIC'.
Can you help me to learn more?
(Other definitions for aerobic that I've seen before include "Exercise that enhances circulation" , "Like exercise that aids the intake of oxygen" , "dependent on oxygen" , "(Of exercises) related to increasing oxygen in blood" , "Pertaining to oxygen" .)