Tiny detail - from motorway, head on to A1 northbound (7)
I believe the answer is:
minutia
'tiny detail' is the definition.
(I've seen this before)
'motorway head on to a1 northbound' is the wordplay.
'motorway' becomes 'MI' (resembles Ma, English motorway).
'head' becomes 'nut' (informal term for somebody's head).
'on' says to put letters next to each other.
'to aa' becomes 'ai' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'northbound' says the letters should be written in reverse.
'ai' in reverse letter order is 'ia'.
'mi'+'nut'+'ia'='MINUTIA'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for minutia that I've seen before include "A small or minor detail" , "Precise detail; small, trivial matter" , "Tiny detail" .)