Painful condition acquired by bum swimming in Italian lake (7)
I believe the answer is:
lumbago
'painful condition' is the definition.
(I've seen this before)
'bum swimming in italian lake' is the wordplay.
'swimming' is an anagram indicator (letters swim around into a new order).
'in' indicates putting letters inside.
'italian lake' becomes 'lago' (I've seen this in another clue).
'bum' is an anagram of 'umb'.
'umb' put within 'lago' is 'LUMBAGO'.
'acquired by' acts as a link.
(Other definitions for lumbago that I've seen before include "Painful condition" , "Rheumatism of lower back" , "Back complaint" , "Pain -- a bum log (anag)" , "Rheumatic pain in the small of the back" .)