Look at the big end - open up (6)
I believe the answer is:
glance
'look at' is the definition.
(I've seen this before)
'big end open up' is the wordplay.
'end' says to take the final letters.
'open up' becomes 'lance' (lancing is a kind of opening up).
The last letter of 'big' is 'g'.
'g'+'lance'='GLANCE'
'the' acts as a link.
(Other definitions for glance that I've seen before include "Swift look" , "Take a brief or hurried look" , "fly off" , "Oblique impact" , "Momentary look" .)