One chap maintaining drink temperature's crucial (9)
I believe the answer is:
important
'crucial' is the definition.
(similar in meaning)
'one chap maintaining drink temperature's' is the wordplay.
'one' becomes 'an' (an apple is one apple).
'chap' becomes 'import' (I can't explain this - if you can you should give a lot more credence to this answer).
'maintaining' means one lot of letters goes inside another (inserted letters are held or maintained).
'drink' means one lot of letters goes inside another.
'temperature' becomes 't' (abbreviation).
'an' going into 'import' is 'imporant'.
'imporant' placed around 't' is 'IMPORTANT'.
Can you help me to learn more?
(Other definitions for important that I've seen before include "Of great significance" , "Main" , "influential" , "Historic" , "Leading" .)