Don't know location of London Bridge? Head for market close to here (6,2)
I believe the answer is:
search me
'don't know' is the definition.
('search me' means 'I don't know')
'location of london bridge? head for market close to here' is the wordplay.
'location of london' becomes 'SE' (postcode for South East of London).
'bridge?' becomes 'arch' (I've seen this before).
'head for' says to take the initial letters.
'close to' suggests the final letters (the 'close' to the word).
The initial letter of 'market' is 'm'.
The final letter of 'here' is 'e'.
'se'+'arch'+'m'+'e'='SEARCH ME'
(Other definitions for search me that I've seen before include "I certainly don't know" , "Examination set on this writer" , "Wearing cashmere? - I don't know, go ahead and frisk" , "I haven't a clue" , "what customs officer might do" .)