Clear evidence of leg injury I had (6)
I believe the answer is:
'clear' is the definition.
(similar in meaning)
'evidence of leg injury i had' is the wordplay.
'evidence of leg injury' becomes 'limp' (someone with an injured leg might limp).
'i had' becomes 'I'd' (I'd).
(Other definitions for limpid that I've seen before include "Clear (writing, water)" , "Crystal clear, transparent" , "Clear, like air or water, say?" , "Crystal clear like pure water" , "Completely clear" .)