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' shows that the letters should be reversed in order.
'bros' in reverse letter order is 'sorb'.
'a'+'b'+'sorb'='ABSORB'
'first' acts as a 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" .)