Unusually achy and tense on boat (5)
I believe the answer is:
yacht
'boat' is the definition.
(yachting is a kind of boating)
'unusually achy and tense' is the wordplay.
'unusually' indicates an anagram.
'and' means one lot of letters go next to another.
'tense' becomes 't' (abbreviation used in many dictionaries).
'achy' with letters rearranged gives 'yach'.
'yach'+'t'='YACHT'
'on' acts as a link.
(Other definitions for yacht that I've seen before include "Sailing-boat" , "Sailing craft" , "Pleasure craft" , "Cathy (anag.)" , "Light, fast sailing-vessel" .)