Small fish covering tons of metal (5)
I believe the answer is:
steel
'metal' is the definition.
(steel is a metal alloy)
'small fish covering tons' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'fish' becomes 'eel' (eel is a type of fish).
'covering' is an insertion indicator.
'tons' becomes 't'.
's'+'eel'='seel'
'seel' placed around 't' is 'STEEL'.
'of' acts as a link.
(Other definitions for steel that I've seen before include "Iron/carbon alloy" , "Strong metal" , "Determination, metal" , "nerve" , "Mixture of carbon and iron" .)