Make longer (6)
'make longer' is the definition.
(extending is a kind of making longer)
This is all the clue.
(Other definitions for extend that I've seen before include "Make larger or longer" , "Provide challenge" , "Carry on" , "Stretch out - enlarge" , "Hold out or stretch out" .)
'make longer' is the definition.
(elongating is a kind of making longer)
This is the entire clue.
(Other definitions for elongate that I've seen before include "Spread" , "Make longer" , "Stretch lengthways" , "Gone late oddly to extend" , "stretcher will do!" .)
'make longer' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for lengthen that I've seen before include "increase in duration" , "Make or become longer" , "Elongate" , "Protract" , "Prolong" .)