Think logically about a brief SOS number (6)
I believe the answer is:
reason
'think logically' is the definition.
(synonyms)
'about a brief sos number' is the wordplay.
'about' becomes 'rea' (I've seen this in other clues).
'a brief' means to remove the last letter (a shorter or briefer version of the word).
'number' becomes 'n' (n can represent any number).
'sos' with its final letter taken away is 'so'.
'rea'+'so'+'n'='REASON'
(Other definitions for reason that I've seen before include "use of wit" , "Incentive" , "issue" , "Sanity of senora" , "Logic; excuse" .)