Customary place to retreat ... an island on a lake (12)
I believe the answer is:
conventional
'customary' is the definition.
(similar in meaning)
'place to retreat an island on a lake' is the wordplay.
'place to retreat' becomes 'convent' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'an island' becomes 'i' (geographical abbreviation).
'lake' becomes 'L' (geographical abbreviation).
'convent'+'i'+'on'+'a'+'l'='CONVENTIONAL'
Can you help me to learn more?
(Other definitions for conventional that I've seen before include "Customary, mainstream" , "Customary; not 9 ac" , "Standard" .)