Harmful microorganism found in bigger meal (4)
I believe the answer is:
germ
'harmful microorganism' is the definition.
(I've seen this in another clue)
'in bigger meal' is the wordplay.
'in' says the answer is hidden in the clue.
'GERM' can be found hidden inside 'bigger meal'.
'found' acts as a link.
(Other definitions for germ that I've seen before include "Harmful micro-organism" , "Microbe, bacterium" , "Infecting microorganism" , "Bug" , "Micro-organism that causes disease" .)