Leader of unbelievers locks up a biblical figure (5)
I believe the answer is:
uriah
'a biblical figure' is the definition.
(I've seen this before)
'leader of unbelievers locks up' is the wordplay.
'leader of' says to take the initial letters.
'locks' becomes 'hair' (locks can mean hair).
'up' says the letters should be written backwards (in down clue: letters go upwards).
The first letter of 'unbelievers' is 'u'.
'hair' in reverse letter order is 'riah'.
'u'+'riah'='URIAH'
(Other definitions for uriah that I've seen before include "Dickensian hypocrite ... Heep" , "King David had him killed in battle in order to marry his wife, Bathsheba" , "'Umble man from Dickens (first name)" , "Dickens hypocrite ..... Heep" , "Biblical figure" .)