A foot in a line
I believe the answer is:
iamb
'a foot in a line' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for iamb that I've seen before include "Part of a pentameter?" , "Metrical foot of one short, one long syllables" , "Short-long metric foot" , "Short-long metrical foot" , "Metrical unit" .)