Exercise involving oxygen 50/50 with nitrogen, this helps growth (6)
I believe the answer is:
pollen
'this helps growth' is the definition.
(pollen helps plants grow)
'exercise involving oxygen 5050' is the wordplay.
'exercise' becomes 'pen' (I can't justify this - if you can you should give a lot more credence to this answer).
'involving' indicates putting letters inside.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'5050' becomes 'LL' (two Ls, Roman numerals for 50).
'o'+'ll'='oll'
'pen' going around 'oll' is 'POLLEN'.
'with nitrogen' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for pollen that I've seen before include "What sticks to insects" , "Dust-like powder from some plants" , "Fine powder given off by plants" , "Frequent cause of allergies" , "Powder from flowers" .)