Birdie regularly, eagle periodically - perfect! (5)
I believe the answer is:
ideal
'perfect' is the definition.
(similar in meaning)
'birdie regularly eagle periodically' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'periodically' means one should take alternating letters (letters must be selected 'every so often').
The alternate letters of 'birdie' are 'ide'.
The alternating letters of 'eagle' are 'al'.
'ide'+'al'='IDEAL'
(Other definitions for ideal that I've seen before include "In her dreams" , "Pattern" , "fanciful" , "Nonpareil" , "Perfect but not likely to become a reality" .)