Spacious yacht's first to tie up, heading north (5)
I believe the answer is:
roomy
'spacious' is the definition.
('roomy' can be a synonym of 'spacious')
'yacht's first to tie up heading north' is the wordplay.
'first' says to take the initial letters.
'to tie up' becomes 'moor' (I've seen this before).
'heading north' says the letters should be written in reverse.
The initial letter of 'yachts' is 'y'.
'y'+'moor'='ymoor'
'ymoor' back-to-front is 'ROOMY'.
(Other definitions for roomy that I've seen before include "Broad" , "Spacious, can swing a cat in it" , "so wide" , "Voluminous" , "Without cramping" .)