Muscle problem (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">muscle problem</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for strain that I've seen before include "Overtax; tension" , "Music" , "Muscle injury, put liquid through sieve" , "Twist" , "tax" .)