Small fish makes horrid pong (6)
I believe the answer is:
stench
'horrid pong' is the definition.
'stench' can be an answer for 'pong' (I've seen this before). I am not certain of the 'horrid' bit.
'small fish' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fish' becomes 'tench' (tench is a kind of fish).
's'+'tench'='STENCH'
'makes' is the link.
Can you help me to learn more?
(Other definitions for stench that I've seen before include "Nasty smell" , "Horrid smell" , "Dreadful pong" , "smell, to put it mildly" , "Terrible smell" .)