Go by boat around north, but it's terribly slow (5)
I believe the answer is:
snail
'it's terribly slow' is the definition.
I can't judge whether this defines the answer.
'go by boat around north' is the wordplay.
'go by boat' becomes 'sail' (I've seen this in another clue).
'around' is an insertion indicator.
'north' becomes 'N' (abbreviation).
'sail' enclosing 'n' is 'SNAIL'.
'but' is the link.
Can you help me to learn more?
(Other definitions for snail that I've seen before include "Gastropod mollusc" , "Slow-moving garden pest" , "dawdler" , "Its speed was emulated by Shakespeare's creeping schoolboy" , "French delicacy" .)