Needed by a sickly boy (6)
I believe the answer is:
wanted
'needed' is the definition.
('want' can be a synonym of 'need')
'by a sickly boy' is the wordplay.
'by' is a charade indicator (letters next to each other).
'a sickly' becomes 'wan' (I've seen this in another clue).
'boy' becomes 'Ted' (short for Edward or Theodore).
'wan' put next to 'ted' is 'WANTED'.
(Other definitions for wanted that I've seen before include "Required, needed" , "Desired or wished for" , "Lacked; type of police poster" , "sought by police?" , "Desired, needed" .)