Was effective around left — that bloke's happy (6)
I believe the answer is:
'happy' is the definition.
(I've seen this before)
'was effective around left that bloke's' is the wordplay.
'was effective' becomes 'bit' (I've seen this in other clues).
'around' indicates putting letters inside.
'left' becomes 'l' (common abbreviation).
'that bloke' becomes 'he' ('he' refers to a man).
'bit' placed around 'l' is 'blit'.
(Other definitions for blithe that I've seen before include "Carefree and happy" , "Cheerfully indifferent" , "Gay" , "Happy and maybe casual" , "Uncaring" .)