Sarah is turning up as apostle’s companion (5)
I believe the answer is:
silas
'apostle's companion' is the definition.
The definition suggests a singular noun which matches the answer.
'sarah is turning up' is the wordplay.
'sarah' becomes 'Sal' (Sally is a common pet form of Sarah).
'turning up' shows that the letters should be reversed in order.
'sal'+'is'='salis'
'salis' in reverse letter order is 'SILAS'.
'as' is the link.
Can you help me to learn more?
(Other definitions for silas that I've seen before include "'Uncle . . . . .' , novel by J Sheridan Le Fanu" , "Fellow" , "Missionary" , "Uncle - (Le Fanu); - Marner (Eliot)" , "Chap" .)