Sample detailed plan, altering mine (8)
I believe the answer is:
specimen
'sample' is the definition.
(specimen is a kind of sample)
'detailed plan altering mine' is the wordplay.
'detailed plan' becomes 'spec'.
'altering' indicates anagramming the letters.
'mine' anagrammed gives 'imen'.
'spec'+'imen'='SPECIMEN'
(Other definitions for specimen that I've seen before include "Sample taken for medical testing" , "Example regarded as typical of its class" , "Exemplar" , "blood sample" , "Copy" .)