Perfunctory repast including tea -- largely pleasant (10)
I believe the answer is:
mechanical
'perfunctory' is the definition.
(thesaurus)
'repast including tea largely pleasant' is the wordplay.
'repast' becomes 'meal' (synonyms).
'including' is an insertion indicator.
'tea' becomes 'cha' (cha is a type of tea).
'largely' means to remove the last letter (most but not all of the letters).
'pleasant' becomes 'nice' (associated in meaning).
'nice' with its final letter taken away is 'nic'.
'cha'+'nic'='chanic'
'meal' placed around 'chanic' is 'MECHANICAL'.
(Other definitions for mechanical that I've seen before include "Lacking thought or spontaneity" , "Machine-like" , "Robotic" , "Automatic" , "Bottom perhaps" .)