Brief rest, perhaps, for an egghead (5)
I believe the answer is:
terse
'brief' is the definition.
(synonyms)
'rest perhaps for an egghead' is the wordplay.
'perhaps' is an anagram indicator.
'for' is a charade indicator (letters next to each other).
'an egghead' becomes 'e' (head letter of 'egg').
'rest' with letters rearranged gives 'ters'.
'ters'+'e'='TERSE'
(Other definitions for terse that I've seen before include "Curt, laconic of speech" , "Sententious" , "Reset to be pithy" , "Short, abrupt" , "Brusque and curt" .)