Utterly like a road being repaired by engineers inside arch (9)

'utterly' is the definition.

'like a road being repaired by engineers inside arch' is the wordplay.
'like a road being repaired' becomes 'up' (construction workers might have a road 'up').
'by' is a charade indicator (letters next to each other).
'engineers' becomes 'reme' (I've seen this in other clues).
'inside' means one lot of letters goes inside another.
'arch' becomes 'sly' (I've seen this in other clues).
'up'+'reme'='upreme'
'upreme' going into 'sly' is 'SUPREMELY'.