To get by, I have to be brief and unassertive (7)
I believe the answer is:
passive
'unassertive' is the definition.
(thesaurus)
'to get by i have' is the wordplay.
'to get by' becomes 'pass' (I've seen this in another clue).
'i have' becomes 'I've' (contraction of 'I have').
'pass'+'ive'='PASSIVE'
'to be brief and' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for passive that I've seen before include "sort of voice" , "Slow to react" , "Showing no resistance" , "Unreactive" , "Opposite of active" .)