Crack one with line did you say? (7)
I believe the answer is:
fissure
'crack' is the definition.
(I know that crack can be written as fissure)
'one with line did you say?' is the wordplay.
'one with line' becomes 'fisher' (someone fishing with a line).
'did you say?' shows a homophone (sound like).
'fisher' is a homophone of 'FISSURE'.
(Other definitions for fissure that I've seen before include "Cleft" , "Deep crack in rock" , "Long, narrow crack" , "Long, narrow opening" , "Split - cleavage" .)