Slowly advanced over top of obstacle (5)
I believe the answer is:
lento
'slowly' is the definition.
(musical instruction)
'advanced over top of obstacle' is the wordplay.
'advanced' becomes 'lent' (advance can mean to lend).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'top of' suggests taking the first letters.
The first letter of 'obstacle' is 'o'.
'lent'+'o'='LENTO'
(Other definitions for lento that I've seen before include "Instruction to players" , "How one scores slowly" , "(When playing music) slowly" , "It's slower than adagio" , "Musically low speed" .)