Give in as say, snot's brought up containing cold bacteria's source (7)
I believe the answer is:
succumb
'give in' is the definition.
(synonyms)
'snot's brought up containing cold bacteria's source' is the wordplay.
'snot' becomes 'mucus' (I've seen this before).
'brought up' is a reversal indicator (in a down clue, letters go up).
'containing' is an insertion indicator.
'cold' becomes 'C' (eg on taps).
'source' suggests taking the first letters (source can mean a beginning or origin).
The initial letter of 'bacterias' is 'b'.
'mucus' back-to-front is 'sucum'.
'sucum' enclosing 'c' is 'succum'.
'succum'+'b'='SUCCUMB'
'as say' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for succumb that I've seen before include "Gives in under pressure, capitulates" , "Give in (to temptation etc)" , "Give way to something overpowering" , "Give in or fall victim to" , "Be fatally overwhelmed" .)