Choice of prudish sweetheart (5)
I believe the answer is:
prime
'choice' is the definition.
(I know that choice can be written as prime)
'prudish sweetheart' is the wordplay.
'prudish' becomes 'prim' (synonyms).
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'prim'+'e'='PRIME'
'of' is the link.
(Other definitions for prime that I've seen before include "Main - charge" , "Two or three, for example" , "Maybe eleven" , "Prepare for use, say a pump" , "(Of number) divisible only by itself and one" .)