Notice youngster at wedding making escape (7)
I believe the answer is:
seepage
'escape' is the definition.
(I've seen this before)
'notice youngster at wedding' is the wordplay.
'notice' becomes 'see' (I've seen this before).
'youngster at wedding' becomes 'page' (young boy attending a bride).
'see'+'page'='SEEPAGE'
'making' is the link.
(Other definitions for seepage that I've seen before include "narrow escape?" , "Slow fluid loss" , "Slow flow of fluid" , "Ooze" , "Percolation" .)