Uniquely, advanced at half pace (4)
I believe the answer is:
once
'uniquely' is the definition.
(once can mean one time or uniquely)
'advanced at half pace' is the wordplay.
'advanced at' becomes 'on' (location. I am not sure about the 'advanced' bit.).
'half' indicates taking half.
'pace' with half the letters taken is 'ce'.
'on'+'ce'='ONCE'
Can you help me to learn more?
(Other definitions for once that I've seen before include "In days past" , "Previously" , "In the old days" , "One-time musical film and song" , "Singly" .)