Flexible poem about tail-wagging (9)
I believe the answer is:
versatile
'flexible' is the definition.
(similar in meaning)
'poem about tail-wagging' is the wordplay.
'poem' becomes 'verse' (verse is a kind of poem).
'about' is an insertion indicator.
'wagging' indicates anagramming the letters (letters swing or wag around).
'tail' with letters rearranged gives 'atil'.
'verse' going around 'atil' is 'VERSATILE'.
(Other definitions for versatile that I've seen before include "Capable of many functions" , "Adaptable sort of relatives" , "Relatives able to do many things" , "Adapting easily to different tasks" , "Good at adapting oneself" .)