Apart from one, trainees developed promise (7)
I believe the answer is:
earnest
'promise' is the definition.
(I've seen this before)
'apart from one trainees developed' is the wordplay.
'apart from' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'developed' indicates an anagram.
'trainees' with 'i' taken out is 'tranees'.
'tranees' is an anagram of 'EARNEST'.
(Other definitions for earnest that I've seen before include "Art worthy" , "'Heartfelt, sincere (7)'" , "promise" , "Intensely serious; token" , "Sincere; determined" .)