Thought fun is late starting (6)
I believe the answer is:
musing
'thought' is the definition.
(I know that musing is a type of consideration)
'fun is late starting' is the wordplay.
'fun' becomes 'amusing' (synonyms).
'is late starting' means to remove the first letter (the word starts at its second letter).
'amusing' with its first letter taken away is 'MUSING'.
(Other definitions for musing that I've seen before include "Reflecting or pondering" , "Cogitation" , "Meditating" , "reflection" , "Pondering, thinking" .)