Ultimate plan I'm reticent somewhat to take up (8)
I believe the answer is:
terminal
'ultimate' is the definition.
(thesaurus)
'plan i'm reticent somewhat to take up' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'to take up' shows that the letters should be reversed in order (I've seen 'taken up' mean this).
'lanimret' is hidden in the letters of 'plan im reticent'.
'lanimret' reversed gives 'TERMINAL'.
(Other definitions for terminal that I've seen before include "Fatal - tram line (anag)" , "Final; connection point" , "Computer device" , "Passenger building" , "Leading to death, fatal" .)