Engage in (4)
I believe the answer is:
wage
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">engage in</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for wage that I've seen before include "Salary" , "Regular amount of income" , "something to do with war?" , "Money earned" , "Amount earned" .)