Set out to be a warm-hearted bit of sunshine (5)
I believe the answer is:
array
'set out' is the definition.
(synonyms)
'to be a warm-hearted bit of sunshine' is the wordplay.
'to be a' becomes 'a'.
'warm' becomes 'thermal' (similar in meaning).
'hearted' means to look at the middle letters.
'bit of sunshine' becomes 'ray' (I've seen this before).
The centre of 'thermal' is 'r'.
'a'+'r'+'ray'='ARRAY'
(Other definitions for array that I've seen before include "Marshal" , "Matrix; arrange" , "Impressive arrangement" , "Matrix; elaborate clothing" , "Impressive exhibition" .)