Boast about tier (4)
I believe the answer is:
crow
'boast' is the definition.
(crow is a kind of boast)
'about tier' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa'**).
'tier' becomes 'row' (I've seen this before).
'c'+'row'='CROW'
(Other definitions for crow that I've seen before include "'As the - - flies'" , "Boast gloatingly, American Indian" , "Black bird with raucous call" , "Make sounds of pleasure as a baby does - or a bird" , "Black bird; boast" .)