Fish the Spanish stock online (3)
I believe the answer is:
eel
'fish' is the definition.
(I know that eel is a type of fish)
'the spanish stock online' is the wordplay.
'the spanish' becomes 'el' ('the' in Spanish).
'stock' is an insertion indicator (some letters contain or stock others).
'online' becomes 'e' (as in e-mail or e-commerce).
'el' placed around 'e' is 'EEL'.
(Other definitions for eel that I've seen before include "Long thin fish" , "Conger, e.g." , "Serpentine fish" , "slipper" , "Slimy fish" .)