Tagging a single word with the nltk pos tagger tags each letter instead of the word

nltk.tag.pos_tag accepts a list of tokens, separate and tags its elements. Therefore you need to put your words in an iterable like list:

>>> nltk.tag.pos_tag(['going'])
[('going', 'VBG')]

>>> word = 'going'
>>> word = nltk.word_tokenize(word)
>>> l1 = nltk.pos_tag(word)
>>> l1
[('going', 'VBG')]

Return the POS tag of one word

nltk.pos_tag(["going"])
----->[('going', 'VBG')]