Girl and a monarch go boating (8)
I believe the answer is:
kayaking
'boating' is the definition.
(kayaking is a kind of boating)
'girl and a monarch' is the wordplay.
'girl' becomes 'Kay'.
'and' says to put letters next to each other.
'monarch' becomes 'king' (king is a kind of monarch).
'kay'+'a'+'king'='KAYAKING'
'go' is the link.
(Other definitions for kayaking that I've seen before include "Operating canoe" , "sport" , "Travelling in a small canoe" .)