The pretty redhead's left, but it's not important (5)
I believe the answer is:
petty
'it's not important' is the definition.
I can't judge whether this definition defines the answer.
'the pretty redhead's left' is the wordplay.
'redhead' becomes 'r' (head letter of 'red').
'left' suggests deleting specific letters (some letters have left).
'pretty' with 'r' taken out is 'PETTY'.
'but' is the link.
Can you help me to learn more?
(Other definitions for petty that I've seen before include "This cash is of a trivial amount" , "Trivial, contemptible" , "small" , "Small-time" , "Inferior" .)