A reptile about to strike - I must be agile (9)
I believe the answer is:
acrobatic
'agile' is the definition.
(I've seen this before)
'a reptile about to strike i' is the wordplay.
'reptile' becomes 'croc' (I've seen this before).
'about' means one lot of letters goes inside another.
'to strike' becomes 'bat'.
'bat'+'i'='bati'
'croc' enclosing 'bati' is 'crobatic'.
'a'+'crobatic'='ACROBATIC'
'must be' acts as a link.
(Other definitions for acrobatic that I've seen before include "Like a tumbling or swinging performer" , "Showing agility" , "Like a spectacular gymnast" , "Gymnastically agile" , "Like a gymnastic entertainer" .)