Pierce head of saltwater fish (5)
I believe the answer is:
spike
'pierce' is the definition.
(spiking is a kind of piercing)
'head of saltwater fish' is the wordplay.
'head of' suggests taking the first letters.
'fish' becomes 'pike' (pike is a kind of fish).
The first letter of 'saltwater' is 's'.
's'+'pike'='SPIKE'
(Other definitions for spike that I've seen before include "Sharp increase in intensity" , "ear" , "Long sharp-pointed implement" , "add something to it" , "Unexpected amount of juice" .)