Look at what's on envelope (7)
I believe the answer is:
address
'look at' is the definition.
(I've seen this before)
'on envelope' is the wordplay.
I cannot quite see how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is located in the answer.
This explanation may well be incorrect...
'what's' is the link.
Can you help me to learn more?
(Other definitions for address that I've seen before include "Begin to deal with" , "lecture?" , "directions in window" , "details on envelope" , "Speak to - direct" .)