Looking back, trade in bric-a-brac most unlikely (7)
I believe the answer is:
tallest
'most unlikely' is the definition.
(I've seen this before)
'looking back trade in bric-a-brac' is the wordplay.
'looking back' shows that the letters should be reversed in order.
'trade' becomes 'sell' (I've seen this before).
'in' is an insertion indicator.
'bric-a-brac' becomes 'tat' (I've seen this before).
'sell' put within 'tat' is 'tsellat'.
'tsellat' written backwards gives 'TALLEST'.
(Other definitions for tallest that I've seen before include "highest standing" , "most exaggerated" , "most implausible" , "Sell tat (anag.)" , "Of extreme height" .)