Bad smell of fish at Spithead (6)
I believe the answer is:
stench
'bad smell' is the definition.
(I've seen this before)
'fish at spithead' is the wordplay.
'fish' becomes 'tench' (tench is a kind of fish).
'at' means one lot of letters go next to another (I've seen this in other clues).
'spithead' becomes 's' (head letter of 'spit').
'tench' after 's' is 'STENCH'.
'of' acts as a link.
(Other definitions for stench that I've seen before include "Miasma" , "Evidence of corruption" , "Foul odour" , "A really vile smell" , "smell, to put it mildly" .)