Feebler, frailer (6)
I believe the answer is:
weaker
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">feebler</span>' is the first definition.<br/ ><span class="explanation-format__explanation">('weak' can be similar in meaning to 'feeble')</span><br/ ><br/ >'<span class="explanation-format__original">frailer</span>' is the second definition.<br/ ><span class="explanation-format__explanation">('weak' can be similar in meaning to 'frail')</span><br/ ><br/ ></span>
(Other definitions for weaker that I've seen before include "Less strong" , "More lacking in strength" , "Less powerful" , "More feeble or frail" , "More powerless" .)