Reckon why men adjusted spanner (6,6)
I believe the answer is:
monkey wrench
'spanner' is the definition.
(monkey wrench is a kind of spanner)
'reckon why men adjusted' is the wordplay.
'adjusted' indicates an anagram.
'reckon'+'why'+'men'='reckonwhymen'
'reckonwhymen' with letters rearranged gives 'MONKEY-WRENCH'.
(Other definitions for monkey wrench that I've seen before include "One having flexible jaws" , "tool is needed" , "Large spanner with one adjustable jaw" .)