A boat turning south on every occasion (6)
I believe the answer is:
always
'on every occasion' is the definition.
(I've seen this before)
'a boat turning south' is the wordplay.
'boat' becomes 'yawl' (yawl is a kind of sailing vessel).
'turning' says the letters should be written backwards.
'south' becomes 's' (abbreviation).
'yawl' reversed gives 'lway'.
'a'+'lway'+'s'='ALWAYS'
(Other definitions for always that I've seen before include "Habitually" , "Every time" , "Continually" , "Irving Berlin's enduring song" , "At all times" .)