Concentrated way we count decimally: it’s very easy at first (9)
I believe the answer is:
intensive
'concentrated' is the definition.
(synonyms)
'easy at first' is the wordplay.
'easy' becomes 'tensive' (I am not sure about this - if you are sure you should believe this answer much more).
'at' becomes 'in' (synonymous in some cases - eg at school, in school).
'first' means one lot of letters go next to another.
'tensive' put after 'in' is 'INTENSIVE'.
'way we count decimally it's very' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for intensive that I've seen before include "Demanding" , "Concentrated" , "Very vigorous or thorough; concentrating" .)