Blunder moving along southern borders (4)
I believe the answer is:
lips
'borders' is the definition.
(I know this)
'blunder moving along southern' is the wordplay.
I cannot quite see how this works, but
'southern' could be 's' (abbreviation) and 's' is present in the answer.
The remaining letters 'lip' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for lips that I've seen before include "Olivia's two, 'indifferent red' (T. Night)" , "Take, O take those - away (Measure for Measure)" , "margins" , "Edges of the mouth" , "Brims, rims" .)