Perspicacity one shows at first in period of darkness (7)
I believe the answer is:
insight
'perspicacity' is the definition.
(I've seen this before)
'one shows at first in period of darkness' is the wordplay.
'one' becomes 'I' (Roman numeral).
'at first' indicates taking the first letters.
'in' indicates putting letters inside.
'period of darkness' becomes 'night' (it is dark at night).
The first letter of 'shows' is 's'.
's' going within 'night' is 'nsight'.
'i'+'nsight'='INSIGHT'
(Other definitions for insight that I've seen before include "Deep understanding" , "comprehension" , "Perceptiveness" , "Awareness" , "Practical knowledge" .)