Tiny boy in work is beginning to thrive as cheerful type (8)
I believe the answer is:
optimist
'cheerful type' is the definition.
(I've seen this before)
'tiny boy in work is beginning to thrive' is the wordplay.
'tiny boy' becomes 'Tim' (A Christmas Carol character Tiny Tim).
'in' is an insertion indicator.
'work' becomes 'op' (abbreviation for opus).
'beginning to' indicates taking the first letters.
The initial letter of 'thrive' is 't'.
'op'+'is'='opis'
'tim' inserted inside 'opis' is 'optimis'.
'optimis'+'t'='OPTIMIST'
'as' acts as a link.
(Other definitions for optimist that I've seen before include "Glass-half-full person" , "One whose glass is half full?" , "One always tends to take a favourable view of things" , "One looking on bright side" , "One expecting all to be well" .)