Tailless camel biting earl: it may cause a complaint (8)
I believe the answer is:
bacteria
'it may cause a complaint' is the definition.
(bacteria might cause a medical complaint)
'tailless camel biting earl' is the wordplay.
'tailless' means to remove the last letter (the tail of the word is removed).
'camel' becomes 'Bactrian camel' (Bactrian camel is an example).
'biting' means one lot of letters goes inside another (some letters bite or swallow others).
'earl' becomes 'e'.
'bactrian' with its last letter taken away is 'bactria'.
'bactria' placed around 'e' is 'BACTERIA'.
(Other definitions for bacteria that I've seen before include "Tiny causes of disease" , "source of disease" , "Cabaret I found containing microscopic organisms" , "Tiny organisms found in bait race" , "Infectious microorganisms" .)