Monitor, perhaps, triplane I rebuilt (9)
I believe the answer is:
reptilian
'monitor perhaps' is the definition.
(monitor is a kind of reptilian)
'triplane i rebuilt' is the wordplay.
'rebuilt' indicates anagramming the letters.
'triplane'+'i'='triplanei'
'triplanei' with letters rearranged gives 'REPTILIAN'.
(Other definitions for reptilian that I've seen before include "like a lizard" , "Contemptible" , "Like a 27 [MONITOR]" , "Mean" , "Snake-like" .)