Change direction at first when set upon (6)
I believe the answer is:
attack
'set upon' is the definition.
(I've seen this before)
'change direction at first' is the wordplay.
I cannot quite understand how this works, but
'change' could be 'tack' (tack is a kind of change) and 'tack' is found in the answer.
'at' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'when' is the link.
Can you help me to learn more?
(Other definitions for attack that I've seen before include "Set upon" , "storm" , "An assault or onslaught" , "Go on the offensive" , "act of aggression" .)