A line of research (3)

I believe the answer is:

arc

'a line' is the definition.
(arc is a kind of line)

'of research' is the wordplay.
'of' indicates the answer is hidden within the clue.
'ARC' can be found hidden inside 'research'.

(Other definitions for arc that I've seen before include "section of Circle Line" , "Luminous electrical discharge" , "circle, partly" , "Something used to weld" , "development of plot" .)

I've seen this clue in the Evening Standard.

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