Page seven rewritten about one in thought (7)
I believe the answer is:
'thought' is the definition.
'page seven rewritten about one' is the wordplay.
'page' becomes 'p'.
'rewritten' indicates an anagram.
'about' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'seven' anagrammed gives 'ensve'.
'ensve' placed around 'i' is 'ensive'.
'in' acts as a link.
(Other definitions for pensive that I've seen before include "Brooding", "Deeply thoughtful", "Introspective, maybe moody", "in a brown study?", "Thinking deeply".)