Effect a change of tack at sea (5,5)
I believe the answer is:
bring about
'effect' is the definition.
(synonyms)
'a change of tack at sea' is the wordplay.
I cannot really see how this works, but
'a' is found in the answer.
'change' could be 'bring' (bringing is a kind of changing) and 'bring' is present in the answer.
'of' could be 'o' and 'o' is found within the answer.
The remaining letters 'but' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for bring about that I've seen before include "Cause" , "Create" .)