Again, seek to investigate (8)
I believe the answer is:
research
'investigate' is the definition.
(I know that research is a more specific form of the action investigate)
'again seek' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'seek' becomes 'search' (synonyms).
're'+'search'='RESEARCH'
'to' acts as a link.
(Other definitions for research that I've seen before include "Experimentation" , "Look into" , "Systematic investigation to establish facts" , "Investigation designed to expand knowledge" , "Thoroughly investigate" .)