Solitary lecturer covering Fenland city (6)
I believe the answer is:
lonely
'solitary' is the definition.
(synonyms)
'lecturer covering fenland city' is the wordplay.
'lecturer' becomes 'l' (abbreviation for lecturer).
'covering' becomes 'on'.
'fenland city' becomes 'ely' (Ely is a city in the Fens).
'l'+'on'+'ely'='LONELY'
(Other definitions for lonely that I've seen before include "Isolated" , "Feeling a lack of companionship" , "Lacking companions" , "In need of companionship" , "From the beginning (Latin)" .)