Yearly, see a nun swimming in a lake (6)
I believe the answer is:
annual
'yearly' is the definition.
(synonyms)
'a nun swimming in a lake' is the wordplay.
'swimming' is an anagram indicator.
'in' means one lot of letters goes inside another.
'lake' becomes 'L' (geographical abbreviation).
'nun' with letters rearranged gives 'nnu'.
'a' inserted within 'nnu' is 'annu'.
'annu'+'a'+'l'='ANNUAL'
'see' acts as a link.
(Other definitions for annual that I've seen before include "Occurring once every year" , "Hardy it may be" , "Regular publication" , "(Of plant) living for a year or less" , "Occuring once a year" .)