I plan too carelessly, but it's not mandatory (8)
I believe the answer is:
optional
'it's not mandatory' is the definition.
(I've seen this before)
'i plan too carelessly' is the wordplay.
'carelessly' is an anagram indicator.
'i'+'plan'+'too'='iplantoo'
'iplantoo' anagrammed gives 'OPTIONAL'.
'but' acts as a link.
(Other definitions for optional that I've seen before include "Unforced" , "Open to choice - take your pick" , "Not compulsory, discretionary" , "'Left to choice, not compulsory (8)'" , "Such as one can take or leave" .)