Love trapped by one obscure colloquialism (5)
I believe the answer is:
idiom
'colloquialism' is the definition.
(style of speech or idiom used in conversation)
'love trapped by one obscure' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'trapped by' indicates putting letters inside.
'one' becomes 'I' (Roman numeral).
'obscure' becomes 'dim' (both can mean dark).
'i'+'dim'='idim'
'o' going within 'idim' is 'IDIOM'.
(Other definitions for idiom that I've seen before include "Phrase such as over the moon" , "Expression that's characteristic of particular group" , "Linguistic peculiarity" , "Language peculiar to a particular group" , "Characteristic style of a period" .)