Boast by very close relative (5)
I believe the answer is:
vaunt
'boast' is the definition.
(I know that vaunt is a type of boast)
'by very close relative' is the wordplay.
'by' is a charade indicator (letters next to each other).
'very' becomes 'v' (common abbreviation).
'close relative' becomes 'aunt' (I've seen this before).
'v'+'aunt' is 'VAUNT'.
(Other definitions for vaunt that I've seen before include "crow" , "Trumpet" , "Show off" , "Boast about (something), often excessively" , "Talk boastfully of" .)