Developed an aid, in spirit (5)
I believe the answer is:
naiad
'spirit' is the definition.
(I've seen this before)
'developed an aid' is the wordplay.
'developed' indicates an anagram.
'an'+'aid'='anaid'
'anaid' is an anagram of 'NAIAD'.
'in' is the link.
(Other definitions for naiad that I've seen before include "divine female figure" , "Nymph of lakes, springs and rivers" , "Spring nymph" , "Classic spirit" , "Greek mythological water nymph" .)