Needed by a sickly little fellow (6)
I believe the answer is:
wanted
'needed' is the definition.
('want' can be a synonym of 'need')
'by a sickly little fellow' is the wordplay.
'by' is a charade indicator (letters next to each other).
'a sickly' becomes 'wan' (I've seen this before).
'little fellow' becomes 'ted' (I've seen this in other clues. I am not sure about the 'little' bit.).
'wan' put next to 'ted' is 'WANTED'.
Can you help me to learn more?
(Other definitions for wanted that I've seen before include "Required, needed" , "Wished-for" , "Desired by police?" , "Lacked; type of police poster" , "Desired, needed" .)