Most optimistic with one's love rising during sleep (7)
I believe the answer is:
rosiest
'most optimistic' is the definition.
('rosy' can be similar in meaning to 'optimistic')
'one's love rising during sleep' is the wordplay.
'one's' becomes 'is' (roman numeral).
'love' becomes 'o' (In tennis, 'love' means 'zero').
'rising' says the letters should be written in reverse (going up in a down clue).
'during' indicates putting letters inside.
'sleep' becomes 'rest' ('rest' can be a synonym of 'sleep'**).
'is'+'o'='iso'
'iso' reversed gives 'osi'.
'osi' going inside 'rest' is 'ROSIEST'.
'with' acts as a link.
(Other definitions for rosiest that I've seen before include "Most glowing" , "Most optimistic - stories (anag)" , "Most healthy" , "(Of future) most promising; maximally pink" .)