Spectacle seeing insect on leaf (7)
I believe the answer is:
pageant
'spectacle' is the definition.
(I've seen this before)
'insect on leaf' is the wordplay.
'insect' becomes 'ant' (ant is a kind of insect).
'on' means one lot of letters go next to another.
'leaf' becomes 'page' (page is a kind of leaf).
'ant' after 'page' is 'PAGEANT'.
'seeing' is the link.
(Other definitions for pageant that I've seen before include "Parade with tableaux" , "Neat gap for colourful spectacle" , "Tableaux or dramatic scenes depicting historical events" , "Spectacular display" , "entertaining spectacle" .)