Halliday & Matthiessen (1999: 398-9):
One manifestation of the second–order nature of the textual metafunction that is important for our purposes is grammatical metaphor. Grammatical metaphor is a ‘second-order’ use of grammatical resources: one grammatical feature or set of features is used as a metaphor for another feature or set of features; and since features are realised by structures, one grammatical structure comes to stand for another.