Big cat initially gets into line (5)
I believe the answer is:
tiger
'big cat' is the definition.
(I know that tiger is a type of big cat)
'initially gets into line' is the wordplay.
'initially' indicates taking the first letters.
'into' means one lot of letters goes inside another.
'line' becomes 'tier' (tier is a kind of line).
The first letter of 'gets' is 'g'.
'g' placed into 'tier' is 'TIGER'.
(Other definitions for tiger that I've seen before include "Formidable competitor" , "he got his stripes" , "Large wild cat" , "''... ... burning bright, in the forests of the night'' (Blake)" , "Beast as Shere Khan" .)