What does the cryptic crossword clue "Trail grew doubtful, however magnified (4,5)" mean?

The definition is "magnified". The wordplay uses an anagram. anagram of TRAIL + GREW = WRITLARGE; definition: "magnified" The answer is WRITLARGE.

Trail grew doubtful, however magnified (4,5)

Guardian Cryptic #26703 | 5 Down | 2015-10-15 flower