Caesarian section is a strain (4)
I believe the answer is:
aria
'a strain' is the definition.
(aria is a kind of strain)
'caesarian section' is the wordplay.
'section' indicates the answer is hidden within the clue.
'ARIA' can be found hidden inside 'caesarian'.
'is' is the link.
(Other definitions for aria that I've seen before include "Operatic part" , "Repeated strain" , "selection of opera" , "Operatic vocal" , "Imitative art work" .)