Cast first to tackle hard line (5)
I believe the answer is:
throw
'cast' is the definition.
(I know that cast can be written as throw)
'first to tackle hard line' is the wordplay.
'first to' indicates taking the first letters.
'line' becomes 'row' (a row or line of things).
The first letters of 'tackle hard' is 'th'.
'th'+'row'='THROW'
(Other definitions for throw that I've seen before include "Dumbfound" , "shot-put event?" , "Discard" , "Cast, hurl" , "Propel through the air" .)