Deer climb where oxygen’s lacking, initially judging by account (7)

'deer' is the definition.
(muntjac is a kind of deer)

'lacking initially judging by account' is the wordplay.
'lacking' becomes 'munt' (I can't explain this - if you can you should give a lot more credence to this answer).
'initially' indicates taking the first letters.
'by' means one lot of letters go next to another.
'account' becomes 'ac'.
The first letter of 'judging' is 'j'.
'munt'+'j'+'ac'='MUNTJAC'

'climb where oxygen's' is the 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?