Look for a key to open up with (6)
I believe the answer is:
glance
'look' is the definition.
(glance is a kind of look)
'key to open up with' is the wordplay.
I cannot quite understand how this works, but
'key' could be 'g' (musical key) and 'g' is found in the answer.
'open' could be 'lance' (lancing is a kind of opening) and 'lance' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'for a' is the link.
Can you help me to learn more?
(Other definitions for glance that I've seen before include "Butcher's" , "A quick sideways look" , "deflection" , "Take a brief or hurried look" , "A dart of the eye" .)