Take in first two letters before brothers turn up (6)
I believe the answer is:
absorb
'take in' is the definition.
(synonyms)
'two letters before brothers turn up' is the wordplay.
'two letters' means two replacements for 'letter'.
'letter' becomes 'a'.
'letter' becomes 'b' ().
'before' is a charade indicator (letters next to each other).
'brothers' becomes 'bros'.
'turn up' says the letters should be written in reverse.
'bros' reversed gives 'sorb'.
'a'+'b'+'sorb'='ABSORB'
'first' is the link.
(Other definitions for absorb that I've seen before include "Suck up, as a sponge" , "Take or drink in" , "Assimilate or soak up" , "Assimilate, integrate" , "Understand" .)