Change tack at any time, driving leader back (4)
I believe the answer is:
veer
'change tack' is the definition.
'veer' can be an answer for 'tack' (thesaurus). I'm not certain of the 'change' bit.
'at any time driving leader back' is the wordplay.
'at' says to put letters next to each other.
'any time driving' becomes 'vee' (I can't explain this - if you can you should give a lot more credence to this answer).
'back' suggests the final letters.
The final letter of 'leader' is 'r'.
'vee' put next to 'r' is 'VEER'.
(Other definitions for veer that I've seen before include "Change direction or course, like wind" , "Turn sharply" , "Swerve" , "Change direction sharply" , "Swing around" .)