Cat close to falling in line (5)
I believe the answer is:
tiger
'cat' is the definition.
(I know that tiger is a type of cat)
'close to falling in line' is the wordplay.
'close to' says to take the final letters (the close/ending of).
'in' indicates putting letters inside.
'line' becomes 'tier' (tier is a kind of line).
The final letter of 'falling' is 'g'.
'g' inserted into 'tier' is 'TIGER'.
(Other definitions for tiger that I've seen before include "Dynamo" , "audacious person" , "Endangered carnivore" , "One of the big cats" , "Striped big-cat" .)