Person devout ultimately exists for prayer (6)
I believe the answer is:
mantis
'prayer' is the definition.
(I've seen this before)
'person devout ultimately exists' is the wordplay.
'person' becomes 'man' (man is a kind of person).
'ultimately' says to take the final letters.
'exists' becomes 'is' (if something 'is', it exists).
The last letter of 'devout' is 't'.
'man'+'t'+'is'='MANTIS'
'for' acts as a link.
(Other definitions for mantis that I've seen before include "Insect in attitude of prayer" , "Insect of the cockroach family" , "Actually it preys rather than prays" , "Carnivorous insect, which rests as if in prayer" , "Predatory insect" .)