Incomer by ground that has yet to be developed (9)
I believe the answer is:
embryonic
'that has yet to be developed' is the definition.
(embryonic can mean in development)
'incomer by ground' is the wordplay.
'ground' indicates an anagram ('grind' the letters into a new form).
'incomer'+'by'='incomerby'
'incomerby' anagrammed gives 'EMBRYONIC'.
(Other definitions for embryonic that I've seen before include "In the early stages of development" , "in its earliest stages" , "In early stage of development" , "In a rudimentary stage" .)