Stick or hold fast (6)
I believe the answer is:
adhere
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">stick</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(to adhere to something is to stick to it)</span><br/ ><br/ >'<span class="explanation-format__original">hold fast</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for adhere that I've seen before include "'Stick to, a play say (6)'" , "Header (anag)" , "Stick L" , "Stick to as by suction" , "Stick - join" .)