Strengthen second locker wall (7)
I believe the answer is:
bolster
'strengthen' is the definition.
(I've seen this before)
'second locker wall' is the wordplay.
I cannot really see how this works, but
'second' could be 's' (common abbreviation) and 's' is located in the answer.
The remaining letters 'bolter' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for bolster that I've seen before include "Support - strengthen" , "Support; long pillow" , "Hold up" , "Prop up with long pillow" , "Extended pillow" .)