Weak old runner going around in tights (7)
I believe the answer is:
maillot
'tights' is the definition.
(maillot is a kind of tights)
'weak old runner going around' is the wordplay.
'weak' becomes 'ill' (I've seen this before).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'runner going' becomes 'mat' (I am not sure about this - if you are sure you should believe this answer much more).
'around' means one lot of letters goes inside another.
'ill'+'o'='illo'
'illo' placed into 'mat' is 'MAILLOT'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for maillot that I've seen before include "Top" , "Tights worn for ballet" , "Jersey" , "Swimsuit" .)