Coach takes on extra amount (5)
I believe the answer is:
bonus
'extra amount' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'coach takes on' is the wordplay.
'coach' becomes 'bus' (synonyms).
'takes' means one lot of letters goes inside another.
'bus' placed around 'on' is 'BONUS'.
Can you help me to learn more?
(Other definitions for bonus that I've seen before include "Extra payment for good work" , "The advantage" , "Tip" , "Something more than you expected" , "Good performance reward" .)