Stick fast (6)
I believe the answer is:
adhere
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">stick fast</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for adhere that I've seen before include "'Stick to, a play say (6)'" , "Stick to as by suction" , "Stick fast (to something)" , "Keep" , "Stick to or continue faithful" .)