Batty aunt's idea: covering lake with cream (6,6)
I believe the answer is:
suntan lotion
'cream' is the definition.
'batty aunt's idea covering lake' is the wordplay.
'batty' indicates an anagram.
'idea' becomes 'notion' (notion is a kind of idea).
'covering' is an insertion indicator.
'lake' becomes 'l' (geographical abbreviation).
'notion' enclosing 'l' is 'nlotion'.
'aunts' with letters rearranged gives 'sunta'.
'sunta'+'nlotion'='SUNTAN LOTION'
'with' acts as a link.
(Another definition for suntan lotion that I've seen is " Preparation used by sunbathers".)