When spotted, thong is tight (2,5)
I believe the answer is:
on sight
'when spotted' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'thong is tight' is the wordplay.
'tight' is an anagram indicator (tight can mean drunk).
'thong'+'is'='thongis'
'thongis' is an anagram of 'ON SIGHT'.
Can you help me to learn more?
(Other definitions for on sight that I've seen before include "How good musicians can play" , "How to shoot dangerous criminal" , "Without rehearsal performing" , "at time of presentation" , "As soon as presented" .)