A top up (5)
I believe the answer is:
ahead
'up' is the definition.
(similar in meaning)
'a top' is the wordplay.
'top' becomes 'head' (head is a kind of top).
'a'+'head'='AHEAD'
(Other definitions for ahead that I've seen before include "In the leading position" , "at pole" , "advanced" , "In the future" , "What's down the road" .)