Recommended day travel permit to enter Mediterranean island retreat (9)
I believe the answer is:
advisable
'recommended' is the definition.
(similar in meaning)
'day travel permit to enter mediterranean island retreat' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'travel permit' becomes 'visa' (I've seen this before).
'to enter' indicates putting letters inside.
'mediterranean island' becomes 'elba' (Elba is an island of the Mediterranean).
'retreat' says the letters should be written in reverse.
'elba' back-to-front is 'able'.
'd'+'visa'='dvisa'
'dvisa' placed into 'able' is 'ADVISABLE'.
(Other definitions for advisable that I've seen before include "Wise, recommended" , "Prudent - recommended" .)