Expression of gratitude boy uses, swimming around lake (5,3)
I believe the answer is:
bless you
'expression of gratitude' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'boy uses swimming around lake' is the wordplay.
'swimming' indicates an anagram.
'around' means one lot of letters goes inside another.
'lake' becomes 'L' (geographical abbreviation).
'boy'+'uses'='boyuses'
'boyuses' anagrammed gives 'bessyou'.
'bessyou' enclosing 'l' is 'BLESS YOU'.
Can you help me to learn more?
(Other definitions for bless you that I've seen before include "often follows sudden explosion!" , "Prayer" , "as bishop may do?" , "It's said after a sneeze" , "React to one preparing to blow" .)