A lot missing from alternator - it that why one is off course? (6)
I believe the answer is:
errant
'why one is off course?' is the definition.
I can't tell whether this defines the answer.
'a lot missing from alternator it' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
The remaining letters 'err' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'that' acts as a link.
Can you help me to learn more?
(Other definitions for errant that I've seen before include "looking for adventure" , "Rambling" , "Kind of knight who once travelled for adventure" , "so some knights are called" , "Nomadic" .)