I wish it wasn't a mountain expedition - only at first, mind you (4,4)
I believe the answer is:
nota bene
'first mind you' is the definition.
The definition suggests a singular noun which matches the answer.
'i wish it wasn't a mountain expedition only' is the wordplay.
I cannot quite understand how this works, but
'i' could be 'a' and 'a' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
'mountain' could be 'ben' (ben is a kind of mountain) and 'ben' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'at' acts as a link.
Can you help me to learn more?
(Other definitions for nota bene that I've seen before include "NB in full" , "Look carefully" , "'Mark well, in Latin (4,4)'" , "Observe carefully (Latin)" , "Expansion of Latin notation N.B." .)