the cupboard is full of clothes.. how full of clothes can be adjective...? nd what type of adjective is it..?? 

A phrase can also perform the role of an adjective in a sentence. Such phrases are called adjectival phrase. They modify a noun just like an adjective does. 

In the sentence, "The cupboard is full of clothes", the phrase 'full of clothes' describes the noun 'the cupboard'. 

Here, the phrase ‘full of clothes’ is describing the noun ‘the cupboard’. It tells us of what kind the noun is.