Having quickly seen a label, open it up (2,1,6)
I believe the answer is:
at a glance
'having quickly seen a label' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'open it up' is the wordplay.
I cannot quite see how this works, but
'open' could be 'lance' (lancing is a kind of opening) and 'lance' is present in the answer.
'it' could be 'tag' (playground catching game) and 'tag' is found within the answer.
A single letter 'a' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for at a glance that I've seen before include "immediately" , "With just a look" .)