Perfect date in Rome, mostly by a lake (5)
I believe the answer is:
ideal
'perfect' is the definition.
(similar in meaning)
'date in rome mostly by a lake' is the wordplay.
'date in rome' becomes 'ides' (Roman calendar term, as in the Ides of March).
'mostly' means to remove the last letter.
'by' means one lot of letters go next to another.
'lake' becomes 'L' (geographical abbreviation).
'ides' with its last letter taken away is 'ide'.
'ide'+'a'+'l'='IDEAL'
(Other definitions for ideal that I've seen before include "Embodying perfection" , "Standard of perfection or excellence" , "example" , "Just right" , "Best possible" .)