The prime minister entertains little Sarah with a song (5)
I believe the answer is:
psalm
'song' is the definition.
(I've seen this before)
'the prime minister entertains little sarah' is the wordplay.
'the prime minister' becomes 'PM' (abbreviation).
'entertains' means one lot of letters goes inside another.
'little' means to remove the last letter (the word is not quite complete).
'sarah' becomes 'Sall' (Sally is a common pet form of Sarah).
'sall' with its final letter removed is 'sal'.
'pm' placed around 'sal' is 'PSALM'.
'with a' acts as a link.
(Other definitions for psalm that I've seen before include "What's heard in church" , "Sacred song from Old Testament" , "Song - hymn - prayer" , "Religious song" , "Sacred song of praise" .)