At half past, call a number (6)
I believe the answer is:
string
'a number' is the definition.
(I have seen 'Number of racehorses' mean 'string' so perhaps 'number' could also mean 'string')
'at half past call' is the wordplay.
'at' says to put letters next to each other.
'half' indicates taking half.
'call' becomes 'ring' (telephone call).
'past' cut in half is 'st'.
'st'+'ring' is 'STRING'.
(Other definitions for string that I've seen before include "light cord" , "Succession" , "Narrow cord or kind of mesh vest" , "Collectively, the racehorses trained at one stable" , "It's difficult to know how long a piece of it is" .)