Behaviour is alien, quite strange, alien in reverse (9)
I believe the answer is:
etiquette
'behaviour' is the definition.
(I have seen 'Polite behaviour ' mean 'etiquette' so perhaps 'behaviour' could also mean 'etiquette')
'alien quite strange alien in reverse' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'strange' indicates anagramming the letters.
'alien' becomes 'et' (extra terrestrial).
'in reverse' is a reversal indicator.
'et' written backwards gives 'te'.
'quite' anagrammed gives 'iquet'.
'et'+'iquet'+'te'='ETIQUETTE'
'is' acts as a link.
(Other definitions for etiquette that I've seen before include "Convention" , "Rules of correct social behaviour" , "Code of polite behaviour" , "Decorum, ceremony" , "form" .)