Start seeing changes with start of summer (7)
I believe the answer is:
genesis
'start' is the definition.
(thesaurus)
'seeing changes with start of summer' is the wordplay.
'changes with' indicates an anagram.
'start of' indicates taking the first letters.
The first letter of 'summer' is 's'.
'seeing' anagrammed with 's' can make 'GENESIS'.
(Other definitions for genesis that I've seen before include "Old Testament Book 1" , "OT opening" , "Bible's first book" , "Origin (of the Old Testament?)" , "Beginning (in the Bible)" .)