Foolish writer on reflection restricted by drink (5)
I believe the answer is:
inept
'foolish' is the definition.
(thesaurus)
'writer on reflection restricted by drink' is the wordplay.
'writer' becomes 'pen' (I've seen this before).
'on reflection' says the letters should be written in reverse.
'restricted by' indicates putting letters inside (I've seen this in other clues).
'drink' becomes 'it'.
'pen' reversed gives 'nep'.
'nep' put within 'it' is 'INEPT'.
(Other definitions for inept that I've seen before include "Standard required in a piece of work (colloq.)" , "Silly - unskilled - unsuitable" , "Silly or senseless" , "Awkward, showing no skill" , "Unskilful or unsuitable" .)