Mole in crotch, by the sound of it? (6)
I believe the answer is:
groyne
'mole' is the definition.
(protective structure extending into the water)
'crotch by the sound of it?' is the wordplay.
'crotch' becomes 'groin'.
'by the sound of it?' indicates a 'sounds like' (homophone) clue.
'groin' sounds like 'GROYNE'.
'in' is the link.
(Other definitions for groyne that I've seen before include "A bulwark" , "jetty" , "that stands against the tide" , "Sea wall" , "Erosion-control wall" .)