Having gone backward, Spanish car's finally gone forward – fixed! (4-6)
I believe the answer is:
deep-seated
'fixed' is the definition.
(I've seen this before)
'having gone backward spanish car's finally gone forward' is the wordplay.
'having gone' becomes 'peed' (gone to the toilet).
'backward' is a reversal indicator.
'spanish car' becomes 'seat' (I've seen this before).
'finally' suggests the final letters.
The last letters of 'gone forward' are 'ed'.
'peed' back-to-front is 'deep'.
'deep'+'seat'+'ed'='DEEP-SEATED'
(Other definitions for deep-seated that I've seen before include "Firmly fixed" , "Firmly-established" .)