Headless fish blocks weight and tackle (4,2)
I believe the answer is:
take on
'tackle' is the definition.
(synonyms)
'headless fish blocks weight' is the wordplay.
'headless fish' becomes 'ake' (I can't explain this - if you can you should believe this answer much more).
'blocks' means one lot of letters goes inside another (the inside letters form a 'blockage').
'weight' becomes 'ton' (unit of weight).
'ake' going inside 'ton' is 'TAKE ON'.
'and' is the link.
(Other definitions for take on that I've seen before include "Employ or stand up to" , "Agree to undertake" , "Employ - undertake - oppose" , "Become popular; employ" , "recruit" .)