Massive thanks, work unit from the East (5)
I believe the answer is:
great
'massive' is the definition.
(I've seen this in another clue)
'thanks work unit from the east' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'work unit' becomes 'erg' (erg is a kind of work unit).
'from the east' says the letters should be written in reverse (on a map from right to left).
'ta'+'erg'='taerg'
'taerg' written backwards gives 'GREAT'.
(Other definitions for great that I've seen before include "Very good, excellent" , "Marvellous; big" , "Mum's dad's mum, for example" , "Brilliant (like Alfred or Alexander?)" , "and 12 Down: Powerful dog" .)