Extremely tough line for cast (5)
I believe the answer is:
throw
'cast' is the definition.
(synonyms)
'extremely tough line' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'line' becomes 'row' (a row or line of things).
'tough' with its middle removed is 'th'.
'th'+'row'='THROW'
'for' is the link.
(Other definitions for throw that I've seen before include "Form (pot) on a wheel" , "puzzle" , "Organise party" , "Try to lose" , "Cast; give (party)" .)