This, under LA, SF and Santa A, in slippage? (3,7,5)
I believe the answer is:
san andreas fault
'this' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'under la sf and santa a in slippage?' is the wordplay.
'and' says to put letters next to each other.
'in slippage?' is an anagram indicator (I've seen 'in slippage' mean this).
'under'+'la'+'sf'+'santa'+'a'='underlasfsantaa'
'underlasfsantaa' is an anagram of 'SAN ANDREAS FAULT'.
(Another definition for san andreas fault that I've seen is " area of instability".)