Start defending learner driver beginning to turn red (7)
I believe the answer is:
scarlet
'red' is the definition.
(I know this)
'start defending learner driver beginning to turn' is the wordplay.
'start' becomes 'scare' ('you gave me a start!').
'defending' means one lot of letters goes inside another (inserted letters are defended).
'learner driver' becomes 'l' (learner drivers have L-plates on their cars).
'beginning to' suggests taking the first letters.
The initial letter of 'turn' is 't'.
'scare' enclosing 'l' is 'scarle'.
'scarle'+'t'='SCARLET'
(Other definitions for scarlet that I've seen before include "Vivid red" , "Colour of embarrassment?" , "Shade of red" , "Let cars be this colour" , "Red colour of clarets" .)