Not altogether how steam is converted (8)
I believe the answer is:
somewhat
'not altogether' is the definition.
I can't tell whether this definition defines the answer.
'how steam is converted' is the wordplay.
'is converted' is an anagram indicator.
'how'+'steam'='howsteam'
'howsteam' anagrammed gives 'SOMEWHAT'.
Can you help me to learn more?
(Other definitions for somewhat that I've seen before include "Partially, to a degree" , "Rather, a little" , "To a small or moderate degree" , "Moderately" , "To a certain extent" .)