Boatman is into rock (a follower of Yes?) with this instrument (5)
I believe the answer is:
tabor
'this instrument' is the definition.
(tabor is a kind of instrument)
'boatman is into rock a follower of yes?' is the wordplay.
'boatman' becomes 'ab' (abbreviation for able seaman).
'is into' is an insertion indicator.
'rock a follower of yes?' becomes 'tor' (Yes Tor is a rock on Dartmoor).
'ab' put inside 'tor' is 'TABOR'.
'with' is the link.
(Other definitions for tabor that I've seen before include "Mount" , "Type of drum" , "beaten with stick" .)